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Author name code: abbett
ADS astronomy entries on 2022-09-14
author:"Abbett, William P."
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Title: Coupling a Global Heliospheric Magnetohydrodynamic Model to
a Magnetofrictional Model of the Low Corona
Authors: Hayashi, Keiji; Abbett, William P.; Cheung, Mark C. M.;
Fisher, George H.
2021ApJS..254....1H Altcode:
Recent efforts coupling our Sun-to-Earth magnetohydrodynamics (MHD)
model and lower-corona magnetofrictional (MF) model are described. Our
Global Heliospheric MHD (GHM) model uses time-dependent three-component
magnetic field data from the lower-corona MF model as time-dependent
boundary values. The MF model uses data-assimilation techniques to
introduce the vector magnetic field data from the Solar Dynamics
Observatory/Helioseismic and Magnetic Imager, hence as a whole this
simulation coupling structure is driven with actual observations. The
GHM model employs a newly developed interface boundary treatment that
is based on the concept of characteristics, and it properly treats
the interface boundary sphere set at a height of the sub-Alfvénic
lower corona (1.15 R<SUB>⊙</SUB> in this work). The coupled model
framework numerically produces twisted nonpotential magnetic features
and consequent eruption events in the solar corona in response to the
time-dependent boundary values. The combination of our two originally
independently developed models presented here is a model framework
toward achieving further capabilities of modeling the nonlinear
time-dependent nature of magnetic field and plasma, from small-scale
solar active regions to large-scale solar wind structures. This work is
a part of the Coronal Global Evolutionary Model project for enhancing
our understanding of Sun-Earth physics to help improve space weather
capabilities.
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Title: The Coronal Global Evolutionary Model: Using HMI Vector
Magnetogram and Doppler Data to Determine Coronal Magnetic Field
Evolution
Authors: Hoeksema, J. Todd; Abbett, William P.; Bercik, David J.;
Cheung, Mark C. M.; DeRosa, Marc L.; Fisher, George H.; Hayashi, Keiji;
Kazachenko, Maria D.; Liu, Yang; Lumme, Erkka; Lynch, Benjamin J.;
Sun, Xudong; Welsch, Brian T.
2020ApJS..250...28H Altcode: 2020arXiv200614579H
The Coronal Global Evolutionary Model (CGEM) provides data-driven
simulations of the magnetic field in the solar corona to better
understand the build-up of magnetic energy that leads to eruptive
events. The CGEM project has developed six capabilities. CGEM modules
(1) prepare time series of full-disk vector magnetic field observations
to (2) derive the changing electric field in the solar photosphere over
active-region scales. This local electric field is (3) incorporated
into a surface flux transport model that reconstructs a global
electric field that evolves magnetic flux in a consistent way. These
electric fields drive a (4) 3D spherical magnetofrictional (SMF) model,
either at high resolution over a restricted range of solid angles or
at lower resolution over a global domain to determine the magnetic
field and current density in the low corona. An SMF-generated initial
field above an active region and the evolving electric field at the
photosphere are used to drive (5) detailed magnetohydrodynamic (MHD)
simulations of active regions in the low corona. SMF or MHD solutions
are then used to compute emissivity proxies that can be compared
with coronal observations. Finally, a lower-resolution SMF magnetic
field is used to initialize (6) a global MHD model that is driven by
an SMF electric field time series to simulate the outer corona and
heliosphere, ultimately connecting Sun to Earth. As a demonstration,
this report features results of CGEM applied to observations of the
evolution of NOAA Active Region 11158 in 2011 February.
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Title: Modeling a Carrington-scale Stellar Superflare and Coronal
Mass Ejection from {\kappa }^{1}{Cet}
Authors: Lynch, Benjamin J.; Airapetian, Vladimir S.; DeVore,
C. Richard; Kazachenko, Maria D.; Lüftinger, Teresa; Kochukhov,
Oleg; Rosén, Lisa; Abbett, William P.
2019ApJ...880...97L Altcode: 2019arXiv190603189L
Observations from the Kepler mission have revealed frequent
superflares on young and active solar-like stars. Superflares
result from the large-scale restructuring of stellar magnetic
fields, and are associated with the eruption of coronal material
(a coronal mass ejection, or CME) and energy release that can be
orders of magnitude greater than those observed in the largest solar
flares. These catastrophic events, if frequent, can significantly
impact the potential habitability of terrestrial exoplanets through
atmospheric erosion or intense radiation exposure at the surface. We
present results from numerical modeling designed to understand how
an eruptive superflare from a young solar-type star, κ <SUP>1</SUP>
Cet, could occur and would impact its astrospheric environment. Our
data-inspired, three-dimensional magnetohydrodynamic modeling shows
that global-scale shear concentrated near the radial-field polarity
inversion line can energize the closed-field stellar corona sufficiently
to power a global, eruptive superflare that releases approximately
the same energy as the extreme 1859 Carrington event from the Sun. We
examine proxy measures of synthetic emission during the flare and
estimate the observational signatures of our CME-driven shock, both
of which could have extreme space-weather impacts on the habitability
of any Earth-like exoplanets. We also speculate that the observed
1986 Robinson-Bopp superflare from κ <SUP>1</SUP> Cet was perhaps as
extreme for that star as the Carrington flare was for the Sun.
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Title: MHD Simulation of a Superflare and Associated Carrington-Scale
CME Event From the Young Sun
Authors: Lynch, B. J.; Airapetian, V.; Kazachenko, M.; Lueftinger,
T.; DeVore, C. R.; Abbett, W. P.
2018AGUFM.P43H3844L Altcode:
Recent Kepler observations reveal frequent superflares on young
active solar-like stars. We present preliminary simulation results
for a global eruptive flare from the young-Sun analog Kappa-1 Cet. Our
simulation magnetic field initialization is based on a low-order PFSS
representation of the observed stellar magnetogram that provides a
non-trivial dipolar magnetic field configuration with a significantly
warped helmet streamer belt. We use a standard Parker [1958] isothermal
solar wind for the coronal atmosphere and energize the closed-field
stellar corona with idealized shearing flows parallel to the radial
field polarity inversion line. We examine the energy evolution of the
global superflare showing a release of 7.1e+33 erg of magnetic free
energy over the course of 10 hours while the maximum kinetic energy
increase of the CME eruption reaches 2.8e+33 erg, i.e. approximately
the strength of the famous 1859 Carrington Event. We use a flare-ribbon
geometric proxy to calculate a total unsigned flare reconnection flux
of 2.2e+23 Mx and a peak reconnection rate of 8.0e+18 Mx/s. We examine
various proxy measures of synthetic emission during the flare and
discuss the potential for extreme space weather impacts on the early
Earth associated with the CME-driven shock and the CME/ICME flux rope
field structure and orientation.
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Title: Initiation of Superflares and Super-CMEs in Active Solar-type
Stars
Authors: Lynch, B. J.; Airapetian, V. S.; Kazachenko, M. D.;
Lueftinger, T.; DeVore, C. R.; Abbett, W. P.
2018csc..confE..94L Altcode:
Recent Kepler observations reveal frequent superflares on young
active solar-like stars. We present preliminary simulation results
for a global eruptive flare from the young-Sun analog Kappa-1 Cet. Our
simulation magnetic field initialization is based on a low-order PFSS
representation of the observed stellar magnetogram that provides a
non-trivial dipolar magnetic field configuration with a significantly
warped helmet streamer belt. We use a standard Parker [1958] isothermal
solar wind for the coronal atmosphere and energize the closed-field
stellar corona with idealized shearing flows parallel to the radial
field polarity inversion line. We examine the energy evolution of the
global superflare showing a release of 7.1e+33 erg of magnetic free
energy over the course of 10 hours while the maximum kinetic energy
increase of the CME eruption reaches 2.8e+33 erg, i.e. approximately
the strength of the famous 1859 Carrington Event. We use a flare-ribbon
geometric proxy to calculate a total unsigned flare reconnection flux
of 2.2e+23 Mx and a peak reconnection rate of 8.0e+18 Mx/s. We examine
various proxy measures of synthetic emission during the flare and
discuss the potential for extreme space weather impacts on the early
Earth associated with the CME-driven shock and the CME/ICME flux rope
field structure and orientation.
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Title: Excess Lorentz Force in Major Solar Eruptions
Authors: Sun, Xudong; Lynch, Benjamin; Abbett, William; Li, Yan
2018csc..confE..41S Altcode:
The solar active region photospheric magnetic field evolves rapidly
during major eruptive events, suggesting appreciable feedback from
the corona. Using high-cadence vector magnetograms, multi-wavelength
coronal imaging, and numerical simulation, we show how the observed
photospheric "magnetic imprints" are highly structured in space and
time, and how it can in principle be used to estimate the impulse of
the Lorentz force that accelerates the coronal mass ejection (CME)
plasma. In an archetypical event, the Lorentz force correlates well
with the CME acceleration, but the total force impulse surprisingly
exceeds the CME momentum by almost two orders of magnitude. Such a clear
trend exists in about two thirds of the eruptions in our survey for
Cycle 24. We propose a "gentle photospheric upwelling" scenario, where
most of the Lorentz force is trapped in the lower atmosphere layer,
counter-balanced by gravity of the upwelled mass. This unexpected effect
dominates the momentum processes, but is negligible for the energy
budget. We discuss how the upcoming high-sensitivity observations and
new-generation numerical models may help elucidate the problem.
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Title: Using Convection Zone-to-Corona Models to Understand the
Physics of the Solar Wind
Authors: Abbett, W. P.
2015AGUFMSH13E..01A Altcode:
How magnetic energy and flux emerges from the turbulent convective
interior of the Sun into the solar atmosphere is of great importance to
a number of challenging problems in Heliophysics. With the wealth of
data from space-based and ground-based observatories, it is evident
that solar magnetic fields span the entirety of the convection
zone-to-corona system, and do not exist in isolation in a localized
region, or interact only over a prescribed spatial scale. The challenge
of modeling this system in its entirety is that the magnetic field not
only spans multiple scales, but also regions whose physical conditions
vary dramatically. In this overview, I will summarize recent progress
in the effort to dynamically model the upper convection zone-to-corona
system over large spatial scales, and will discuss applications of
these new models to Solar Probe Plus science.
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Title: The Coronal Global Evolutionary Model: Using HMI Vector
Magnetogram and Doppler Data to Model the Buildup of Free Magnetic
Energy in the Solar Corona
Authors: Fisher, G. H.; Abbett, W. P.; Bercik, D. J.; Kazachenko,
M. D.; Lynch, B. J.; Welsch, B. T.; Hoeksema, J. T.; Hayashi, K.;
Liu, Y.; Norton, A. A.; Dalda, A. Sainz; Sun, X.; DeRosa, M. L.;
Cheung, M. C. M.
2015SpWea..13..369F Altcode: 2015arXiv150506018F
The most violent space weather events (eruptive solar flares and
coronal mass ejections) are driven by the release of free magnetic
energy stored in the solar corona. Energy can build up on timescales
of hours to days, and then may be suddenly released in the form of a
magnetic eruption, which then propagates through interplanetary space,
possibly impacting the Earth's space environment. Can we use the
observed evolution of the magnetic and velocity fields in the solar
photosphere to model the evolution of the overlying solar coronal
field, including the storage and release of magnetic energy in such
eruptions? The objective of CGEM, the Coronal Global Evolutionary Model,
funded by the NASA/NSF Space Weather Modeling program, is to develop
and evaluate such a model for the evolution of the coronal magnetic
field. The evolving coronal magnetic field can then be used as a
starting point for magnetohydrodynamic (MHD) models of the corona,
which can then be used to drive models of heliospheric evolution and
predictions of magnetic field and plasma density conditions at 1AU.
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Title: Modeling the Convection Zone-to-Corona System over Global
Spatial Scales
Authors: Abbett, W. P.; Bercik, D. J.; Fisher, G. H.
2014AGUFMSH44A..01A Altcode:
How magnetic energy and flux emerges from the turbulent convective
interior of the Sun into the solar atmosphere is of great importance
to a number of challenging problems in solar physics. With the wealth
of data from missions such as SDO, Hinode, and IRIS, it is evident that
the dynamic interaction of magnetic structures at the photosphere and in
the solar atmosphere occurs over a vast range of spatial and temporal
scales. Emerging active regions often develop magnetic connections
to other regions of activity some distance away on the solar disk,
and always emerge into a global coronal field whose structural
complexity is a function of the solar cycle. Yet even small-scale
dynamic interactions (e.g., processes at granular or supergranular
scales in the photosphere) can trigger rapid changes in the large-scale
coronal field sufficient to power eruptive events such as coronal mass
ejections, or solar flares. The challenge of modeling this system in
its entirety is that the magnetic field not only spans multiple scales,
but also regions whose physical conditions vary dramatically. We will
summarize recent progress in the effort to dynamically model the upper
convection zone-to-corona system over large spatial scales, and will
present the latest results from a new, global radiative-MHD model of the
upper convection zone-to-corona system, RADMHD2S. We will characterize
the flux of electromagnetic energy into the solar atmosphere as flux
systems of different scales dynamically interact, and discuss how
physics-based models of the convection zone-to-corona system can be
used to guide the development and testing of data-driven models.
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Title: Understanding Measures of Magnetic Activity Using Physics-based
Models of the Solar Interior and Atmosphere
Authors: Abbett, W. P.; Luhmann, J. G.
2014AGUFMSH13D4140A Altcode:
Substantial progress has been made over the past decade in the effort
to better understand how magnetic flux and energy is generated
in the convective interior of the Sun, how it emerges into the
solar atmosphere, and how manifestations of solar magnetic activity
(such as sunspots, coronal mass ejections, and flares) are connected
within a dynamic magnetic environment spanning the solar convection
zone-to-corona system. Here, we present a brief overview of recent
efforts to model the evolution of active region magnetic fields and
sunspots over a range of physical conditions and spatial and temporal
scales. We will focus on how dynamic, physics-based numerical models
can be used to better understand observed relationships between
different measures of solar activity as a function of time (e.g.,
sunspot activity and morphologies, unsigned magnetic flux measured at
the photosphere, coronal X-ray emissivity). We will determine whether
local physics-based models of active region evolution can be used
to better constrain proxies of solar activity such as the sunspot
number, which remains the only direct record available to trace the
very long-term influence of the solar dynamo on the earth's environment.
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Title: RADMHD2S: A Global 3D Radiative-MHD Model of the Upper
Convection Zone-to-Corona System
Authors: Abbett, William P.; Bercik, David J
2014AAS...22412347A Altcode:
We present the latest results from a new, global radiative-MHD
model of the upper convection zone-to-corona system, RADMHD2S. The
numerical methods build upon those of the RADMHD model of Abbett
(2007) and Abbett & Fisher (2012), and significantly extend the
capabilities of that code to allow for large-scale, sufficiently
resolved, global calculations over a non-uniform, 3D curvilinear
(spherical) mesh. RADMHD2S utilizes a high-order, non-dimensionally
split, semi-implicit finite volume formalism to update the
system of conservation equations in a way that properly propagates
discontinuities in off-axis directions, while simultaneously preserving
the 3D solenoidal constraint on the magnetic field. In addition, we
will discuss improvements in the treatment of energetics, radiative
transport, and cross-field diffusion that allow for more realistic
data-driven modeling of the model's photosphere and chromosphere.
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Title: Buildup of Magnetic Shear and Free Energy during Flux Emergence
and Cancellation
Authors: Fang, Fang; Manchester, Ward, IV; Abbett, William P.; van
der Holst, Bart
2012ApJ...754...15F Altcode: 2012arXiv1205.3764F
We examine a simulation of flux emergence and cancellation, which shows
a complex sequence of processes that accumulate free magnetic energy
in the solar corona essential for the eruptive events such as coronal
mass ejections, filament eruptions, and flares. The flow velocity at
the surface and in the corona shows a consistent shearing pattern along
the polarity inversion line (PIL), which together with the rotation of
the magnetic polarities, builds up the magnetic shear. Tether-cutting
reconnection above the PIL then produces longer sheared magnetic field
lines that extend higher into the corona, where a sigmoidal structure
forms. Most significantly, reconnection and upward-energy-flux transfer
are found to occur even as magnetic flux is submerging and appears
to cancel at the photosphere. A comparison of the simulated coronal
field with the corresponding coronal potential field graphically shows
the development of non-potential fields during the emergence of the
magnetic flux and formation of sunspots.
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Title: Buildup of Free Energy for Eruptive Events during Flux
Emergence
Authors: Fang, Fang; Manchester, Ward, IV; Abbett, William P.; van
der Holst, Bart
2012shin.confE..39F Altcode:
In a simulation of magnetic flux emergence from the convection zone,
a complex sequence of processes accumulate free magnetic energy in the
solar corona essential for the eruptive events such as coronal mass
ejections (CMEs), filament eruptions and flares. The flow velocity at
the surface and in the corona shows a consistent shearing pattern along
the polarity inversion line (PIL), which together with the rotation of
the magnetic polarities, builds up the magnetic shear. Tether-cutting
reconnection above the PIL then produces longer sheared magnetic field
lines that extend higher into the corona, where a sigmoidal structure
forms. Most significantly, reconnection and upward energy-flux transfer
are found to occur even as magnetic flux is submerging and appears
to cancel at the photosphere. A comparison of the simulated coronal
field with the corresponding coronal potential field graphically shows
the development of non-potential fields during the emergence of the
magnetic flux and formation of sunspots.
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Title: An Improved 3D Radiative-MHD Model of the Convection
Zone-to-Corona System
Authors: Abbett, William P.; Bercik, D. J.; Kazachenko, M.
2012AAS...22020507A Altcode:
We present the latest results from an improved radiative-MHD model of
the convection zone-to-corona system. The numerical methods of the
RADMHD model of Abbett & Fisher (2012) have been significantly
updated so that the underlying finite volume scheme is (1) no longer
dimensionally split along coordinate axes; (2) of much higher order
accuracy using a three-dimensional 27-point stencil; and (3) capable
of performing much larger scale calculations in both spherical polar
coordinates and Cartesian coordinates. We will describe the improvements
of the underlying scheme in detail, present a 3D dynamic convection
zone-to-corona quiet Sun model using the new formalism, and compare
the latest results with previous models.
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Title: A First Look at Magnetic Field Data Products from SDO/HMI
Authors: Liu, Y.; Scherrer, P. H.; Hoeksema, J. T.; Schou, J.; Bai,
T.; Beck, J. G.; Bobra, M.; Bogart, R. S.; Bush, R. I.; Couvidat,
S.; Hayashi, K.; Kosovichev, A. G.; Larson, T. P.; Rabello-Soares,
C.; Sun, X.; Wachter, R.; Zhao, J.; Zhao, X. P.; Duvall, T. L., Jr.;
DeRosa, M. L.; Schrijver, C. J.; Title, A. M.; Centeno, R.; Tomczyk,
S.; Borrero, J. M.; Norton, A. A.; Barnes, G.; Crouch, A. D.; Leka,
K. D.; Abbett, W. P.; Fisher, G. H.; Welsch, B. T.; Muglach, K.;
Schuck, P. W.; Wiegelmann, T.; Turmon, M.; Linker, J. A.; Mikić,
Z.; Riley, P.; Wu, S. T.
2012ASPC..455..337L Altcode:
The Helioseismic and Magnetic Imager (HMI; Scherrer & Schou 2011)
is one of the three instruments aboard the Solar Dynamics Observatory
(SDO) that was launched on February 11, 2010 from Cape Canaveral,
Florida. The instrument began to acquire science data on March 24. The
regular operations started on May 1. HMI measures the Doppler velocity
and line-of-sight magnetic field in the photosphere at a cadence of
45 seconds, and the vector magnetic field at a 135-second cadence,
with a 4096× 4096 pixels full disk coverage. The vector magnetic
field data is usually averaged over 720 seconds to suppress the p-modes
and increase the signal-to-noise ratio. The spatial sampling is about
0".5 per pixel. HMI observes the Fe i 6173 Å absorption line, which
has a Landé factor of 2.5. These data are further used to produce
higher level data products through the pipeline at the HMI-AIA Joint
Science Operations Center (JSOC) - Science Data Processing (Scherrer et
al. 2011) at Stanford University. In this paper, we briefly describe the
data products, and demonstrate the performance of the HMI instrument. We
conclude that the HMI is working extremely well.
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Title: Generation of electric currents via neutral-ion drag in the
chromosphere and ionosphere
Authors: Krasnoselskikh, V.; Abbett, W. P.; Hudson, H.; Vekstein,
G.; Bale, S. D.
2012AIPC.1439...42K Altcode:
We consider the generation of electric currents in the solar
chromosphere. The ionization level in this region is generally supposed
to be low. We show that the ambient electrons are magnetized even
for weak magnetic fields (30 G), i.e. their gyrofrequency is larger
than the collision frequency; ion motions continue to be dominated by
ion-neutral collisions in this region. Under such conditions the ions
are dragged by neutrals. As a result, the dynamics of magnetic field
resembles frozen-in motion of the field with the neutral gas. On the
other hand magnetized electrons drift under the action of the electric
and magnetic fields induced in the reference frame of ions moving with
the neutral gas. This relative motion of electrons and ions results in
the generation of quite intense electric currents. The dissipation of
these currents leads to the resistive electron heating and efficient
gas ionization. Ionization by electron-neutral impact does not alter
the dynamics of the heavy particles; thus the gas turbulent motions
persist even when the plasma becomes fully ionized and the resistive
current dissipation continues to heat electrons and ions. This heating
process is so efficient that it can result in typical temperature
increases with altitude as large as 0.1-0.3 eV/km. We conclude that this
process can play a major role in the heating of the chromosphere and
corona. We show that the physical conditions in the solar chromosphere,
in particular the neutral and ion density dependencies upon altitude,
are very similar to those in the lower ionosphere of the Earth. A
very similar process of current generation occurs in the ionosphere
after strong earthquakes, resulting in the generation of strong
perturbations in the ionosphere. We then present well-known results of
the observations of such perturbations, which allow an evaluation of the
increment of the growth of the perturbations with altitude, making use
of ionospheric sounding. These results are in perfect agreement with
estimates obtained making use a model similar to ours. We consider
that these observations clearly show the efficiency of the physical
mechanisms discussed, and thus provide strong support for our ideas.
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Title: The Impact of the Chromosphere on Numerical Models of the
Convection Zone-to-Corona System
Authors: Abbett, W. P.
2012decs.confE..52A Altcode:
This review will provide an overview of recent progress toward
simulating the magnetic and energetic connection between the convection
zone and corona with a particular emphasis on the effect of the
chromosphere on the coupled system. We will discuss the challenges
inherent in modeling the dynamics and energetics of the chromosphere,
then review what 3D MHD simulations of the atmosphere are able to tell
us about about the transport of magnetic flux and energy from below the
visible surface into the low atmosphere and corona. We will explore how
the dynamic chromosphere affects the structure and non-potentiality
of the overlying coronal field, and what implications this may have
to force-free models based on photospheric magnetograms.
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Title: Radiative Cooling in MHD Models of the Quiet Sun Convection
Zone and Corona
Authors: Abbett, W. P.; Fisher, G. H.
2012SoPh..277....3A Altcode: 2011arXiv1102.1035A
We present a series of numerical simulations of the quiet-Sun plasma
threaded by magnetic fields that extend from the upper convection zone
into the low corona. We discuss an efficient, simplified approximation
to the physics of optically thick radiative transport through the
surface layers, and investigate the effects of convective turbulence
on the magnetic structure of the Sun's atmosphere in an initially
unipolar (open field) region. We find that the net Poynting flux below
the surface is on average directed toward the interior, while in the
photosphere and chromosphere the net flow of electromagnetic energy is
outward into the solar corona. Overturning convective motions between
these layers driven by rapid radiative cooling appears to be the source
of energy for the oppositely directed fluxes of electromagnetic energy.
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Title: Can We Determine Electric Fields and Poynting Fluxes from
Vector Magnetograms and Doppler Measurements?
Authors: Fisher, G. H.; Welsch, B. T.; Abbett, W. P.
2012SoPh..277..153F Altcode: 2011arXiv1101.4086F
The availability of vector-magnetogram sequences with sufficient
accuracy and cadence to estimate the temporal derivative of the magnetic
field allows us to use Faraday's law to find an approximate solution
for the electric field in the photosphere, using a Poloidal-Toroidal
Decomposition (PTD) of the magnetic field and its partial time
derivative. Without additional information, however, the electric
field found from this technique is under-determined - Faraday's law
provides no information about the electric field that can be derived
from the gradient of a scalar potential. Here, we show how additional
information in the form of line-of-sight Doppler-flow measurements,
and motions transverse to the line-of-sight determined with ad-hoc
methods such as local correlation tracking, can be combined with the
PTD solutions to provide much more accurate solutions for the solar
electric field, and therefore the Poynting flux of electromagnetic
energy in the solar photosphere. Reliable, accurate maps of the Poynting
flux are essential for quantitative studies of the buildup of magnetic
energy before flares and coronal mass ejections.
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Title: Momentum Distribution in Solar Flare Processes
Authors: Hudson, H. S.; Fletcher, L.; Fisher, G. H.; Abbett, W. P.;
Russell, A.
2012SoPh..277...77H Altcode:
We discuss the consequences of momentum conservation in processes
related to solar flares and coronal mass ejections (CMEs), in particular
describing the relative importance of vertical impulses that could
contribute to the excitation of seismic waves ("sunquakes"). The
initial impulse associated with the primary flare energy transport
in the impulsive phase contains sufficient momentum, as do the
impulses associated with the acceleration of the evaporation flow (the
chromospheric shock) or the CME itself. We note that the deceleration
of the evaporative flow, as coronal closed fields arrest it, will tend
to produce an opposite impulse, reducing the energy coupling into
the interior. The actual mechanism of the coupling remains unclear
at present.
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Title: Electric Fields and Poynting Fluxes from Vector Magnetograms
Authors: Fisher, G. H.; Welsch, B. T.; Abbett, W. P.
2012decs.confE..75F Altcode:
The availability of vector-magnetogram sequences with sufficient
accuracy and cadence to estimate the temporal derivative of the magnetic
field allows us to use Faraday's law to find an approximate solution
for the electric field in the photosphere, using a Poloidal-Toroidal
Decomposition (PTD) of the magnetic field and its partial time
derivative. Without additional information, however, the electric
field found from this technique is under-determined - Faraday's law
provides no information about the electric field that can be derived
from the gradient of a scalar potential. Here, we show how additional
information in the form of line-of-sight Doppler-flow measurements,
and motions transverse to the line-of-sight determined with ad-hoc
methods such as local correlation tracking, can be combined with the
PTD solutions to provide much more accurate solutions for the solar
electric field, and therefore the Poynting flux of electromagnetic
energy in the solar photosphere. Reliable, accurate maps of the Poynting
flux are essential for quantitative studies of the buildup of magnetic
energy before flares and coronal mass ejections.
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Title: Dynamic Coupling of Convective Flows and Magnetic Field during
Flux Emergence
Authors: Fang, Fang; Manchester, Ward, IV; Abbett, William P.; van
der Holst, Bart
2012ApJ...745...37F Altcode: 2011arXiv1111.1679F
We simulate the buoyant rise of a magnetic flux rope from the solar
convection zone into the corona to better understand the energetic
coupling of the solar interior to the corona. The magnetohydrodynamic
model addresses the physics of radiative cooling, coronal heating,
and ionization, which allow us to produce a more realistic model
of the solar atmosphere. The simulation illustrates the process by
which magnetic flux emerges at the photosphere and coalesces to form
two large concentrations of opposite polarities. We find that the
large-scale convective motion in the convection zone is critical to
form and maintain sunspots, while the horizontal converging flows
in the near-surface layer prevent the concentrated polarities from
separating. The footpoints of the sunspots in the convection zone
exhibit a coherent rotation motion, resulting in the increasing
helicity of the coronal field. Here, the local configuration of the
convection causes the convergence of opposite polarities of magnetic
flux with a shearing flow along the polarity inversion line. During
the rising of the flux rope, the magnetic energy is first injected
through the photosphere by the emergence, followed by energy transport
by horizontal flows, after which the energy is subducted back to the
convection zone by the submerging flows.
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Title: Roles for Data Assimilation in Studying Solar Flares & CMEs
Authors: Welsch, B. T.; Abbett, W. P.; Fisher, G. H.
2011AGUFMSH54A..05W Altcode:
Solar flares and coronal mass ejections (CMEs) are driven by the
sudden release of free magnetic energy stored in electric currents
the solar corona. While there is a consensus that free energy enters
the corona from the solar interior, there is ongoing debate about the
physical processes primarily responsible for transporting free energy
into the corona and / or triggering its release once there. Since
direct measurements of the coronal vector magnetic field, necessary
to quantify coronal currents, are currently not feasible, it is hoped
that modeling of the coronal field can improve our understanding
of processes that drive the corona to flare and produce CMEs. Many
coronal modeling efforts employ spectropolarimetric observations
of the photosphere, which can be used to infer magnetic fields and
flows there; the model then relates these photospheric measurements
to coronal currents. Observations of coronal emission structures
might also usefully inform coronal field models. Here, I will discuss
different approaches to modeling the coronal magnetic field, using
both photospheric and other data sets, and possible roles for data
assimilation.
---------------------------------------------------------
Title: Can we Determine Electric Fields and Poynting Fluxes from
Vector Magnetograms and Doppler Measurements?
Authors: Fisher, G. H.; Welsch, B. T.; Abbett, W. P.
2011AGUFMSH33C..07F Altcode:
The availability of vector magnetogram sequences with sufficient
accuracy and cadence to estimate the time derivative of the magnetic
field allows us to use Faraday's law to find an approximate solution
for the electric field in the photosphere, using a Poloidal-Toroidal
Decomposition (PTD) of the magnetic field and its partial time
derivative. Without additional information, however, the electric
field found from this technique is under-determined -- Faraday's law
provides no information about the electric field that can be derived the
gradient of a scalar potential. Here, we show how additional information
in the form of line-of-sight Doppler flow measurements, and motions
transverse to the line-of-sight determined with ad-hoc methods such as
local correlation tracking, can be combined with the PTD solutions to
provide much more accurate solutions for the solar electric field,
and therefore the Poynting flux of electromagnetic energy in the
solar photosphere. Reliable, accurate maps of the Poynting flux are
essential for quantitative studies of the buildup of magnetic energy
before flares and coronal mass ejections. This work was supported by the
NASA Heliophysics Theory Program, the NASA Living-With-a-Star Program,
and the NSF Geosciences Directorate
---------------------------------------------------------
Title: The Effect of Subsurface Flows during Flux Emergence
Authors: Fang, F.; Manchester, W. B.; Abbett, W. P.; van der Holst, B.
2011AGUFMSH54A..06F Altcode:
Here we carry out magnetohydrodynamic simulations on the emergence of
a buoyant magnetic flux rope through a realistic convection zone that
extends 21 Mm below the photosphere and 21 Mm up into the corona, with
solar thermodynamic processes taken into account. The total maximum
magnetic flux at the photosphere reaches 6.85±10<SUP>20</SUP> Mx,
of the same order of magnitude of solar pores. The main aim of the
simulations is to study the mechanism of the energy and magnetic flux
transfer during the interaction between the subphotospheric flows and
the rising magnetic flux rope. The magnetic flux emerges as bipoles
on the photospheric and subphotospheric layers, then the bipoles
are quickly pulled apart by the horizontal flows and concentrate
in downdrafts. The coalescence of the small-scale bipoles and
convective collapse in the near surface layers form the large-scale
concentrated magnetic flux, i.e. solar pores. The horizontal flow
also exhibits a coherent pattern of rotation, which extends into the
convection zone. Vertical flow in the convection zone pushes down
the endpoints of the flux rope and maintains the bipolar pores during
the emergence. Analysis of the Poynting energy fluxes associated with
vertical and horizontal flows shows that horizontal flow is the main
contributor to the energy transfer from the convection into the corona,
with a value of 6.78±10<SUP>31</SUP> ergs at the photosphere within
8 hours.
---------------------------------------------------------
Title: Observational Analysis of Photospheric Magnetic Field
Restructuring During Energetic Solar Flares
Authors: Alvarado, J. D.; Buitrago, J. C.; Martinez Oliveros, J.;
Lindsey, C. A.; Abbett, W. P.; Fisher, G. H.
2011AGUFMSH13B1944A Altcode:
The magnetic field has proven to be the main driver in the behavior,
dynamics and evolution of several solar atmospheric phenomena including
sunspots, plages, faculae, CME's and flares. Observational evidence of
photospheric magnetic field restructuring during energetic flares have
shown an enhancement of the transversal field component suggesting
an apparent relation between this process with the generation of
“sunquakes”, expanding ripples on the solar photosphere as a
result of the momentum-energy transfer into the solar photosphere
and subphotosphere. In this work we present a doppler and magnetic
observational study of some recent energetic flaring events (X and
M type of the 24th solar cycle) trying to find possible acoustic
signatures and make a characterization of the photospheric magnetic
field evolution during those flares, being this the observational
basis of a future numerical modeling of the field restructuring during
this phenomenon.
---------------------------------------------------------
Title: Coupling of Convective Flows and Emerging Magnetic Fields
Authors: Fang, Fang; Manchester, Ward, IV; Abbett, William P.; van
der Holst, Bart
2011sdmi.confE..31F Altcode:
We carry out radiative MHD simulations on the rising process of a
buoyant magnetic flux rope inside the convection zone and its further
emergence into the upper atmosphere. Our model takes into account
of the radiative cooling, coronal heating and the ionization. The
emergence of the flux rope is accompanied by turbulent surrounding
plasma flows. Analysis on the magnetic fluxes shows that the convective
downflows play an important role in formation of the concentrated
polarities in the convection zone. During the rising of the flux
rope, the magnetic energy is first injected through the photosphere
by the emergence, followed by energy transport by horizontal flows,
after which the energy is subducted back to the convection zone by
the submerging flows.
---------------------------------------------------------
Title: Radiative Cooling in MHD Models of the Quiet Sun Convection
Zone and Corona
Authors: Abbett, William; Fisher, George
2011shin.confE..10A Altcode:
We present a series of numerical simulations of the quiet Sun plasma
threaded by magnetic fields that extend from the upper convection zone
into the low corona. We discuss an efficient, simplified approximation
to the physics of optically thick radiative transport through the
surface layers, and investigate the effects of convective turbulence
on the magnetic structure of the Sun's atmosphere in an initially
unipolar (open field) region. We find that the net Poynting flux below
the surface is on average directed toward the interior, while in the
photosphere and chromosphere the net flow of electromagnetic energy is
outward into the solar corona. Overturning convective motions between
these layers driven by rapid radiative cooling appears to be the source
of energy for the oppositely directed fluxes of electromagnetic energy.
---------------------------------------------------------
Title: Can We Determine Electric Fields and Poynting Fluxes from
Vector Magnetograms and Doppler Measurements?
Authors: Fisher, George H.; Welsch, B. T.; Abbett, W. P.
2011SPD....42.1717F Altcode: 2011BAAS..43S.1717F
The availability of vector-magnetogram sequences with sufficient
accuracy and cadence to estimate the temporal derivative of the magnetic
field allows us to use Faraday's law to find an approximate solution
for the electric field in the photosphere, using a Poloidal--Toroidal
Decomposition (PTD) of the magnetic field and its partial time
derivative. Without additional information, however, the electric
field found from this technique is under-determined -- Faraday's law
provides no information about the electric field that can be derived
the gradient of a scalar potential. Here, we show how additional
information in the form of line-of-sight Doppler-flow measurements,
and motions transverse to the line-of-sight determined with ad-hoc
methods such as local correlation tracking, can be combined with the
PTD solutions to provide much more accurate solutions for the solar
electric field, and therefore the Poynting flux of electromagnetic
energy in the solar photosphere. Reliable, accurate maps of the Poynting
flux are essential for quantitative studies of the buildup of magnetic
energy before flares and coronal mass ejections.
---------------------------------------------------------
Title: Modeling the Physical Connection Between the Solar Convection
Zone and Corona
Authors: Abbett, William P.
2011SPD....42.0101A Altcode: 2011BAAS..43S.0101A
How magnetic energy and flux emerges from below the surface into the
solar atmosphere is a topic ripe for theoretical and observational
investigation, particularly in the SDO era. Data from this mission
is showing us that magnetic fields from the interior emerge through
the surface, and energize the dynamic chromosphere and corona over
a wide range of spatial and temporal scales. The interplay between
granular-scale magnetic features, and large-scale structures from
decaying active regions, for example, are seen to affect the large-scale
solar magnetic field in complex ways. Being able to model these
interactions in a way that captures the disparate spatial and temporal
scales of the convection zone-to-corona system while simultaneously
allowing for direct comparison with observations would be of enormous
value in the effort to better understand the physics of coronal heating,
the energetics of the solar wind, and the onset of magnetic eruptions
(among other phenomena). In this lecture, I will summarize current
progress in the effort to model the magnetic and energetic connection
between the solar interior and atmosphere, and will describe the
limitations and challenges inherent to this holistic approach.
---------------------------------------------------------
Title: Generation of Electric Currents in the Chromosphere via
Neutral-Ion Drag
Authors: Krasnoselskikh, V.; Vekstein, G.; Hudson, H. S.; Bale, S. D.;
Abbett, W. P.
2010ApJ...724.1542K Altcode: 2010arXiv1011.5834K
We consider the generation of electric currents in the solar
chromosphere where the ionization level is typically low. We show that
ambient electrons become magnetized even for weak magnetic fields (30
G); that is, their gyrofrequency becomes larger than the collision
frequency while ion motions continue to be dominated by ion-neutral
collisions. Under such conditions, ions are dragged by neutrals,
and the magnetic field acts as if it is frozen-in to the dynamics of
the neutral gas. However, magnetized electrons drift under the action
of the electric and magnetic fields induced in the reference frame of
ions moving with the neutral gas. We find that this relative motion of
electrons and ions results in the generation of quite intense electric
currents. The dissipation of these currents leads to resistive electron
heating and efficient gas ionization. Ionization by electron-neutral
impact does not alter the dynamics of the heavy particles; thus, the
gas turbulent motions continue even when the plasma becomes fully
ionized, and resistive dissipation continues to heat electrons and
ions. This heating process is so efficient that it can result in
typical temperature increases with altitude as large as 0.1-0.3 eV
km<SUP>-1</SUP>. We conclude that this process can play a major role
in the heating of the chromosphere and corona.
---------------------------------------------------------
Title: Simulation of Flux Emergence in Solar Active Regions
Authors: Fang, F.; Manchester, W. B.; Abbett, W. P.; van der Holst,
B.; Schrijver, C. J.
2010AGUFMSH31A1781F Altcode:
We present results of magnetohydrodynamic (MHD) simulations of
magnetic flux emergence from the convection zone into the solar
corona using BATSRUS. The MHD equations are modified to take account
of the radiative terms, coronal heating and heat conduction. The
implementation of non-ideal equation of state describes the partially
ionized plasma in the convection zone. The simulations are carried out
on a domain of active-region size of 30×30×40 Mm3, extending 20 Mm
down into the convection zone. The magnetic fields are coupled with
the convective motion during the emerging process, and concentrates
in the downflow regions. A coherent shear pattern is formed in the
lower corona during the rising. We also compare our model results
at the photosphere with SDO/HMI vector magnetograms and illustrate
the mechanism of flux emergence that give rise to complexity of the
structures in active regions.
---------------------------------------------------------
Title: Generation of electric currents in the chromosphere via
neutral-ion drag
Authors: Krasnoselskikh, V.; Vekstein, G.; Hudson, H. S.; Bale, S.;
Abbett, W. P.
2010AGUFMSH31C1810K Altcode:
We consider the generation of electric currents in the solar
chromosphere. The ionization level in this region is generally supposed
to be low. We show that the ambient electrons become magnetized even for
weak magnetic fields (30 G), i.e. their gyrofrequency becomes larger
than the collision frequency; ion motions continue to be dominated by
ion-neutral collisions in this region. Under such conditions the ions
are dragged by neutrals and magnetic field dynamics resembles frozen-in
motion of the field with the neutral gas. On the other hand magnetized
electrons drift under the action of the electric and magnetic fields
induced in the reference frame of ions moving with the neutral gas. This
relative motion of electrons and ions results in the generation of quite
intense electric currents. The dissipation of these currents leads to
the resistive electron heating and efficient gas ionization. Ionization
by electron-neutral impact does not alter the dynamics of the heavy
particles; thus the gas turbulent motions continue even when the plasma
becomes fully ionized and the resistive current dissipation continues
to heat electrons and ions. This heating process is so efficient that
it can result in typical temperature increases with altitude as large
as 0.1-0.3 eV/km. We conclude that this process can play a major role
in the heating of the chromosphere and corona.
---------------------------------------------------------
Title: A Simplified Treatment of Radiative Transfer in Large-scale
Convection Zone-to-Corona Models
Authors: Abbett, William P.; Fisher, G. H.
2010shin.confE...6A Altcode:
We present the latest in a series of numerical simulations of quiet
Sun magnetic fields that extend from the upper convection zone into
the low corona. We apply an efficient, simplified treatment of the
physics of optically-thick radiative transfer throughout the surface
layers, and investigate the effects of convective turbulence on the
magnetic structure of the Sun's upper atmosphere in an initially
unipolar (open-field) region. We then compare these results with
earlier simulations that use an ad-hoc, parameterized treatment of
surface cooling.
---------------------------------------------------------
Title: Simulation of Flux Emergence from the Convection Zone to
the Corona
Authors: Fang, Fang; Manchester, Ward; Abbett, William P.; van der
Holst, Bart
2010ApJ...714.1649F Altcode: 2010arXiv1003.6118F
Here, we present numerical simulations of magnetic flux buoyantly
rising from a granular convection zone into the low corona. We study
the complex interaction of the magnetic field with the turbulent
plasma. The model includes the radiative loss terms, non-ideal
equations of state, and empirical corona heating. We find that
the convection plays a crucial role in shaping the morphology and
evolution of the emerging structure. The emergence of magnetic fields
can disrupt the convection pattern as the field strength increases,
and form an ephemeral region-like structure, while weak magnetic flux
emerges and quickly becomes concentrated in the intergranular lanes,
i.e., downflow regions. As the flux rises, a coherent shear pattern
in the low corona is observed in the simulation. In the photosphere,
both magnetic shearing and velocity shearing occur at a very sharp
polarity inversion line. In a case of U-loop magnetic field structure,
the field above the surface is highly sheared while below it is relaxed.
---------------------------------------------------------
Title: Determing Flow Fields Consistent with Vector Magnetic Evolution
Authors: Welsch, Brian; Fisher, G. H.; Abbett, W. P.; Bercik, D. J.
2010AAS...21640112W Altcode: 2010BAAS...41..858W
Sequences of photospheric vector magnetograms can be used to drive
time-dependent models of magnetic evolution in the overlying atmosphere,
as well as to investigate dynamics in the atmospheric layer imaged
in the magnetograms. While several methods of estimating electric
fields consistent with the observed evolution of the magnetic field
normal to the magnetogram surface have been developed, these do not
explicitly employ evolution of the horizontal field components in
deriving electric fields. The recently developed poloidal- toroidal
decomposition (PTD) method (Fisher et al. 2010) does use this extra
information; PTD electric fields, however, are generally not ideal,
so ideality must be imposed post facto. Here, we present formalism
for deriving ideal electric fields consistent with vector magnetic
evolution, assuming that the induction equation in the MHD approximation
governs the magnetic evolution; accordingly, we term the approach
"inductive vector driving", or IVD. This formalism can incorporate
explicit resistive terms. Moreover, IVD allows direct inclusion of
results from tracking methods, which can provide additional information
regarding photospheric evolution. This work is supported by NASA's
Heliophysics Theory Program and NSF's SHINE program.
---------------------------------------------------------
Title: Assimilating Measurements of the Photospheric Magnetic Field
into MHD Models of the Solar Atmosphere
Authors: Abbett, William P.; Fisher, G. H.; Welsch, B. T.; Bercik,
D. J.
2010AAS...21640502A Altcode: 2010BAAS...41..889A
We introduce a rudimentary assimilative technique that allows a time
series of vector magnetograms to be directly incorporated into the
active cells of a RADMHD model of the solar atmosphere. We apply this
technique to a simplified large-scale model of NOAA AR-8210, a flare
and CME-producing active region. We begin by relaxing an initial
magnetic configuration based on the first in a series of IVM vector
magnetograms from the Mees Solar Observatory at Haleakala HI. This
low-beta, near force-free configuration is achieved by solving the
MHD system in the presence of a time-dependent artificial damping
that is reduced as the configuration relaxes. Once the magnetic
and thermodynamic initial state is achieved, we advance the system
using our assimilative technique applied to a 4 hour sequence of IVM
magnetograms. In addition, we present a simple 3D MHD simulation of
the response of the initial AR-8210 pre-flare model corona to flare
energy deposited in the upper chromosphere near a sheared neutral line.
---------------------------------------------------------
Title: Estimating Electric Fields from Vector Magnetogram Sequences
Authors: Fisher, George H.; Welsch, B. T.; Abbett, W. P.; Bercik, D. J.
2010AAS...21640113F Altcode: 2010BAAS...41..859F
Determining the electric field distribution on the Sun's photosphere
is essential for quantitative studies of how energy flows from the
Sun's photosphere, through the corona, and into the heliosphere. This
electric field also provides valuable input for data-driven models of
the solar atmosphere and the Sun-Earth system. We show how observed
vector magnetogram time series can be used to estimate the photospheric
electric field. Our method uses a "poloidal-toroidal decomposition"
(PTD) of the time derivative of the vector magnetic field. These
solutions provide an electric field whose curl obeys all three
components of Faraday's Law. The PTD solutions are not unique; the
gradient of a scalar potential can be added to the PTD electric field
without affecting consistency with Faraday's Law. We then present an
iterative technique to determine a potential function consistent with
ideal MHD evolution; but this field is also not a unique solution
to Faraday's Law. Finally, we explore a variational approach that
minimizes an energy functional to determine a unique electric field,
a generalization of Longcope's "Minimum Energy Fit". The PTD technique,
the iterative technique, and the variational technique are used to
estimate electric fields from a pair of synthetic vector magnetograms
taken from an MHD simulation; and these fields are compared with the
simulation's known electric fields. The PTD and iteration techniques
compare favorably to results from existing velocity inversion
techniques. These three techniques are then applied to a pair of vector
magnetograms of solar active region NOAA AR8210, to demonstrate the
methods with real data.
---------------------------------------------------------
Title: Estimating Electric Fields from Vector Magnetogram Sequences
Authors: Fisher, G. H.; Welsch, B. T.; Abbett, W. P.; Bercik, D. J.
2010ApJ...715..242F Altcode: 2009arXiv0912.4916F
Determining the electric field distribution on the Sun's photosphere
is essential for quantitative studies of how energy flows from the
Sun's photosphere, through the corona, and into the heliosphere. This
electric field also provides valuable input for data-driven models of
the solar atmosphere and the Sun-Earth system. We show how observed
vector magnetogram time series can be used to estimate the photospheric
electric field. Our method uses a "poloidal-toroidal decomposition"
(PTD) of the time derivative of the vector magnetic field. These
solutions provide an electric field whose curl obeys all three
components of Faraday's Law. The PTD solutions are not unique; the
gradient of a scalar potential can be added to the PTD electric field
without affecting consistency with Faraday's Law. We then present an
iterative technique to determine a potential function consistent with
ideal MHD evolution; but this field is also not a unique solution
to Faraday's Law. Finally, we explore a variational approach that
minimizes an energy functional to determine a unique electric field,
a generalization of Longcope's "Minimum Energy Fit." The PTD technique,
the iterative technique, and the variational technique are used to
estimate electric fields from a pair of synthetic vector magnetograms
taken from an MHD simulation; and these fields are compared with the
simulation's known electric fields. The PTD and iteration techniques
compare favorably to results from existing velocity inversion
techniques. These three techniques are then applied to a pair of vector
magnetograms of solar active region NOAA AR8210, to demonstrate the
methods with real data. Careful examination of the results from all
three methods indicates that evolution of the magnetic vector by itself
does not provide enough information to determine the true electric field
in the photosphere. Either more information from other measurements,
or physical constraints other than those considered here are necessary
to find the true electric field. However, we show it is possible to
construct physically reasonable electric field distributions whose curl
matches the evolution of all three components of B. We also show that
the horizontal and vertical Poynting flux patterns derived from the
three techniques are similar to one another for the cases investigated.
---------------------------------------------------------
Title: Improving large-scale convection-zone-to-corona models.
Authors: Abbett, W. P.; Fisher, G. H.
2010MmSAI..81..721A Altcode: 2010arXiv1005.0641A
We introduce two new methods that are designed to improve the realism
and utility of large, active region-scale 3D MHD models of the solar
atmosphere. We apply these methods to RADMHD, a code capable of modeling
the Sun's upper convection zone, photosphere, chromosphere, transition
region, and corona within a single computational volume. We first
present a way to approximate the physics of optically-thick radiative
transfer without having to take the computationally expensive step
of solving the radiative transfer equation in detail. We then briefly
describe a rudimentary assimilative technique that allows a time series
of vector magnetograms to be directly incorporated into the MHD system.
---------------------------------------------------------
Title: Incorporating Magnetogram Data into Time-Dependent Coronal
Field Models
Authors: Fisher, G. H.; Abbett, W. P.; Bercik, D. J.; McTiernan,
J. M.; Welsch, B. T.
2009AGUFMSM51A1340F Altcode:
We briefly review our efforts to incorporate sequences of photospheric
vector magnetograms into MHD simulations of coronal evolution, in an
effort to create data-driven models of the coronal magnetic field. Such
models should improve our understanding of flares and coronal mass
ejections (CMEs), and might eventually lead to predictive capabilities.
---------------------------------------------------------
Title: 3D simulation of flux emergence from convective zone to corona
with BATSRUS
Authors: Fang, F.; Manchester, W. B.; Abbett, W. P.; van der Holst, B.
2009AGUFMSH41B1656F Altcode:
To study the interaction between magnetic field and convective
motion, we present a 3d simulation of the emergence of magnetic flux
ropes from the convective zone into the corona, applying radiation
terms and non-ideal equation of state table to BATSRUS code. To
perform this simulation, we first generate a solar atmosphere,
whose physical properties are comparable with Bercik(2002) data,
with a turbulent convective zone. The upgoing convective motion is
cooled down and stopped by the surface loss and sharp temperature
decrease at photosphere. The magnetic flux is observed to concentrate
at the intergranular lanes with downflowing plasma and decrease in
the granules. We then heat up the corona to temperature of above 1MK,
using an empirical relationship between heating and unsigned magnetic
flux. In the high-temperature, low-density upper atmosphere, radiative
loss term is approximated with optically thin limit and the radiative
cooling curve is obtained from CHIANTI database. The Field aligned heat
conduction is applied to channel heat flux along the magnetic field
lines in corona and form a more realistic transition region. After
producing a superadiabatic atmosphere matching the observed properties,
we introduce a buoyant magnetic flux rope below the photosphere. The
flux rope shows shear flow with velocity of 8km/s at the photosphere
where it emerges. We then compare our results with previous simulations
without convection (Manchester et al. 2004).
---------------------------------------------------------
Title: Estimating Electric Fields from Sequences of Vector
Magnetograms
Authors: Fisher, George H.; Welsch, Brian T.; Abbett, William P.;
Bercik, David J.
2009shin.confE..10F Altcode:
We describe a new technique for estimating the three-dimensional vector
electric field in the solar atmosphere by using a time-sequence
of vector magnetograms to find an electric field distribution
that obeys all 3 components of Faraday's law. The technique uses a
"poloidal-toroidal" decomposition (PTD) to describe the electric field
in terms of two scalar functions. The "inductive" PTD solutions to
Faraday's Law are not unique, however, since additional contributions to
the electric field from a potential function have no effect on Faraday's
law. <P />We then describe how estimates for the total electric field
including both the inductive and potential components can be made
by using variational techniques. The variational approach we develop
is similar to Longcope's "Minimum Energy Fit" technique, in that the
electric field obeys the vertical component of the magnetic induction
equation, while also minimizing a positive definite functional. The
purely potential part of the electric field can then be recovered by
subtracting the PTD electric field from the total field.
---------------------------------------------------------
Title: Estimating Electric Fields from Vector Magnetogram Sequences
Authors: Fisher, George H.; Welsch, B. T.; Abbett, W. P.; Bercik, D. J.
2009SPD....40.0605F Altcode:
We describe a new technique for estimating the three-dimensional vector
electric field in the solar atmosphere by using a time-sequence
of vector magnetograms to find an electric field distribution
that obeys all 3 components of Faraday's law. The technique uses a
“poloidal-toroidal” decomposition (PTD) to describe the electric field
in terms of two scalar functions. The “inductive” PTD solutions to
Faraday's Law are not unique, however, since additional contributions to
the electric field from a potential function have no effect on Faraday's
law. <P />We then describe how estimates for the total electric field
including both the inductive and potential components can be made by
using variational techniques. The variational approach we develop is
similar to Longcope's “Minimum Energy Fit” technique, in that the
electric field obeys the vertical component of the magnetic induction
equation, while also minimizing a positive definite functional. The
purely potential part of the electric field can then be recovered by
subtracting the PTD electric field from the total field.
---------------------------------------------------------
Title: The Dynamic Evolution of Quiet Sun Magnetic Fields
Authors: Abbett, William P.; Fisher, G. H.
2009SPD....40.0903A Altcode:
We present the latest in a series of numerical simulations of
quiet Sun magnetic fields. The upper convection zone, photosphere,
chromosphere, transition region, and corona are all included within a
single computational domain that is sufficiently large to encompass
a typical active region. We introduce a simplified treatment of the
physics of optically-thick radiative transfer throughout the surface
layers, and compare these results with an earlier, non-physics based
parameterized treatment of radiative cooling.
---------------------------------------------------------
Title: Erratum: "Tests and Comparisons of Velocity-Inversion
Techniques" (ApJ, 670, 1434 [2007])
Authors: Welsch, B. T.; Abbett, W. P.; DeRosa, M. L.; Fisher, G. H.;
Georgoulis, M. K.; Kusano, K.; Longcope, D. W.; Ravindra, B.; Schuck,
P. W.
2008ApJ...680..827W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Dynamic Evolution of Active Region Magnetic Fields in
the Solar Atmosphere
Authors: Abbett, W. P.; Fisher, G. H.; Welsch, B. T.; Bercik, D. J.
2008AGUSMSH31A..08A Altcode:
We present the latest results from a series of three-dimensional MHD
simulations of active region magnetic fields. The computational domain
extends from the upper convection zone out into the corona, and includes
the highly-stratified layers of the photosphere, chromosphere, and
transition region. We characterize the effects of convective turbulence
on large-scale magnetic structures, the magnetic connectivity between
sub-surface and coronal fields, and the energetics of the low atmosphere
and corona.
---------------------------------------------------------
Title: Using Ideal Electric Fields Estimated from Vector Magnetogram
Sequences to Drive Coronal MHD Simulations
Authors: Welsch, B. T.; Fisher, G. H.; Abbett, W. P.; Bercik, D. J.
2008AGUSMSH54A..04W Altcode:
Dynamic models of the coronal magnetic field show promise as space
weather forecasting tools. Such models should be driven by electric
fields derived from sequences of photospheric vector magnetograms,
the only routine measurements of the solar magnetic field currently
available. Previous studies derived flows --- or, equivalently, ideal
electric fields --- consistent with evolution of the normal photospheric
field, which could be used in "component driving" of an MHD model,
i.e., enforcing consistent evolution of the observed and modeled normal
magnetic fields. In this extension of the component-driving approach,
we demonstrate how to derive ideal electric fields consistent with the
observed evolution of both the normal and horizontal magnetic field,
useful for "vector driving," i.e., enforcing consistency between all
three components of the observed and model photospheric magnetic
vectors. To drive an MHD model, this "ideal vector driving" (IVD)
approach amount to specification of both the velocity (perpendicular
the magnetic field) and its vertical derivative at the model's bottom
boundary. The IVD method can incorporate results from local/ tracking
methods (e.g., LCT or DAVE) and/or results from global methods (e.g.,
MEF or poloidal-toroidal decomposition [PTD]). We have applied this new
approach to "synthetic magnetograms" extracted from MHD simulations
(where the magnetic and electric fields are exactly known), as well
as to a four-hour sequence of vector magnetograms from NOAA AR 8210,
on 01 May 1998, just prior to an M-class flare and geoeffective CME.
---------------------------------------------------------
Title: Inferring Photospheric Velocity Fields Using a Combination of
Minimum Energy Fit, Local Correlation Tracking, and Doppler Velocity
Authors: Ravindra, B.; Longcope, D. W.; Abbett, W. P.
2008ApJ...677..751R Altcode:
The minimum energy fit (MEF), a velocity inversion technique, infers
all components of the photospheric velocity that are consistent with
the induction equation. From the set of consistent velocity fields,
it selects the smallest overall flow speed by minimizing a kinetic
energy functional. If partial velocity information is available from
other measurements, it can be incorporated into the MEF methodology
by minimizing the squared difference from that data. We incorporate
the partial velocity information provided by local correlation
tracking (LCT) technique and Doppler velocity measurements. We
test the incorporation of these auxiliary velocity fields using the
simulated magnetograms and velocitygrams. To the known velocity field
we compare the results obtained from the MEF alone, the MEF with LCT
constraints, and the MEF with LCT and Doppler information. We find
that the combination of MEF with LCT and vertical velocity yields
the best agreement. We also apply these three methods to actual vector
magnetograms of AR 8210 obtained by the Imaging Vector Magnetograph. The
results suggest that in this active region the helicity and energy
fluxes are dominated by the horizontal rather than the vertical
components of the velocity.
---------------------------------------------------------
Title: Connecting the Quiet-Sun Convection Zone and Corona
Authors: Abbett, W. P.
2008ASPC..383..327A Altcode:
We present the first results of a new numerical model designed to
simultaneously evolve the upper convection zone and low-corona within
a single computational domain. We characterize (1) the properties of
a quiet-Sun model atmosphere that forms as a result of the action of a
convective dynamo; (2) the efficacy of parameterized cooling as a means
of approximating the physics of optically-thick radiative transfer in
the model chromosphere; (3) the magnetic and thermodynamic properties
of the quiet-Sun atmosphere, and the magnetic connectivity between
the turbulent sub-surface layers and corona; and (4) the properties
of horizontally-directed magnetic fields in the low atmosphere.
---------------------------------------------------------
Title: Tests and Comparisons of Velocity-Inversion Techniques
Authors: Welsch, B. T.; Abbett, W. P.; De Rosa, M. L.; Fisher, G. H.;
Georgoulis, M. K.; Kusano, K.; Longcope, D. W.; Ravindra, B.; Schuck,
P. W.
2007ApJ...670.1434W Altcode:
Recently, several methods that measure the velocity of magnetized
plasma from time series of photospheric vector magnetograms have been
developed. Velocity fields derived using such techniques can be used
both to determine the fluxes of magnetic energy and helicity into the
corona, which have important consequences for understanding solar
flares, coronal mass ejections, and the solar dynamo, and to drive
time-dependent numerical models of coronal magnetic fields. To date,
these methods have not been rigorously tested against realistic,
simulated data sets, in which the magnetic field evolution and
velocities are known. Here we present the results of such tests
using several velocity-inversion techniques applied to synthetic
magnetogram data sets, generated from anelastic MHD simulations of
the upper convection zone with the ANMHD code, in which the velocity
field is fully known. Broadly speaking, the MEF, DAVE, FLCT, IM, and
ILCT algorithms performed comparably in many categories. While DAVE
estimated the magnitude and direction of velocities slightly more
accurately than the other methods, MEF's estimates of the fluxes of
magnetic energy and helicity were far more accurate than any other
method's. Overall, therefore, the MEF algorithm performed best in
tests using the ANMHD data set. We note that ANMHD data simulate
fully relaxed convection in a high-β plasma, and therefore do not
realistically model photospheric evolution.
---------------------------------------------------------
Title: The Magnetic Connection between the Convection Zone and Corona
in the Quiet Sun
Authors: Abbett, W. P.
2007ApJ...665.1469A Altcode:
To understand the dynamic, magnetic, and energetic connection
between the convectively unstable layers below the visible surface
of the Sun and the overlying solar corona, we have developed
a new three-dimensional magnetohydrodynamic code capable of
simultaneously evolving a model convection zone and corona within a
single computational volume. As a first application of this numerical
model, we present a series of simulations of the quiet Sun in a domain
that encompasses both the upper convection zone and low corona. We
investigate whether the magnetic field generated by a convective
surface dynamo can account for some of the observed properties of
the quiet-Sun atmosphere. We find that (1) it is possible to heat
a model corona to X-ray-emitting temperatures with the magnetic
fields generated from a convective dynamo and an empirically based
heating mechanism consistent with the observed relationship between
X-ray emission and magnetic flux observed at the visible surface;
(2) within the limitations of our numerical models of the quiet Sun,
resistive and viscous dissipation alone are insufficient to maintain
a hot corona; (3) the quiet-Sun model chromosphere is a dynamic,
non-force-free layer that exhibits a temperature reversal in the
convective pattern in the relatively low density layers above the
photosphere; (4) the majority of the unsigned magnetic flux lies
below the model photosphere in the convectively unstable portion of
the domain; (5) horizontally directed magnetic structures thread the
low atmosphere, often connecting relatively distant concentrations
of magnetic flux observed at the surface; and (6) low-resolution
photospheric magnetograms can significantly underestimate the amount
of unsigned magnetic flux threading the quiet-Sun photosphere.
---------------------------------------------------------
Title: The Dynamic Evolution of Quiet Sun Magnetic Fields in the
Solar Atmosphere
Authors: Abbett, William P.
2007AAS...210.9109A Altcode: 2007BAAS...39..205A
We present the latest results from a series of three-dimensional
MHD simulations of the Quiet Sun magnetic field. The computational
domain extends from the upper convection zone out into the corona, and
includes the highly-stratified layers of the photosphere, chromosphere,
and transition region. Our study focuses on the following questions:
Can the magnetic field generated by a convective surface dynamo heat a
model corona to soft X-ray emitting temperatures? Do physical processes
such as resistive and viscous dissipation supply the necessary heating
throughout the model atmosphere, or is an additional empirically-based
heating mechanism required? What is the magnetic connection between
the flux threading the model photosphere, and that filling the model
corona? Can the magnetic field in the dynamic models be successfully
reproduced by static extrapolations?
---------------------------------------------------------
Title: Active Region Magnetic Fields in the Solar Interior
Authors: Abbett, W. P.; Fisher, G. H.
2006ASPC..354..135A Altcode:
We present a brief review of recent efforts to understand the life-cycle
of active region magnetic fields with an emphasis on what photospheric
observations can tell us about the evolution of large-scale magnetic
structures deep in the convective interior. A critical component of
these efforts is to understand the dynamic connection between magnetic
fields (at both large and small scales) observed threading the solar
atmosphere and their sub-surface counterparts. We conclude our survey
by presenting early results from a new numerical model capable of
self-consistently incorporating both sub-photospheric layers and the
low solar corona into a single computational domain.
---------------------------------------------------------
Title: Are Convective Dynamos Responsible for the Minimum X-ray
Fluxes Observed in the Sun and Late-Type Main Sequence Stars?
Authors: Bercik, D. J.; Fisher, G. H.; Johns-Krull, C. M.; Abbett,
W. P.; Lundquist, L. L.
2006ASPC..354..127B Altcode:
We extend the investigation of tet{Bercik05} to the case of a
non-rotating solar-type reference star. Using three-dimensional
numerical simulations of a turbulent dynamo driven by convection and
the empirical relationship of tet{Pevtsov03}, we predict the level of
X-ray emission from such a convective turbulent dynamo, and find that
our results are consistent with quiet Sun observations. This implies
that it is plausible that the Sun may have a rotation-independent
convective dynamo working together with the large-scale dynamo believed
to be responsible for the solar cycle.
---------------------------------------------------------
Title: Simulations of the Quiet Sun Magnetic Field: From the Upper
Convection Zone into the Corona
Authors: Abbett, William P.
2006SPD....37.0701A Altcode: 2006BAAS...38..227A
We present the latest in a series of simulationsdesigned to directly
investigate whether the magneticfield generated by a convective dynamo
in the upperconvection zone can account for the observedproperties
of the Quiet Sun magnetic field andatmosphere. The simulations are
performed usinga new numerical code capable of evolving a modelsolar
atmosphere that extends from the upper convectionzone into the
low corona. This code is a parallel,semi-implicit solver capable
of accomodating thespatial and temporal disparities intrinsic to
thiscombined system.
---------------------------------------------------------
Title: Radiative Hydrodynamic Models of Optical and Ultraviolet
Emission from M Dwarf Flares
Authors: Allred, Joel C.; Hawley, Suzanne L.; Abbett, William P.;
Carlsson, Mats
2006ApJ...644..484A Altcode: 2006astro.ph..3195A
We report on radiative hydrodynamic simulations of M dwarf stellar
flares and compare the model predictions to observations of several
flares. The flares were simulated by calculating the hydrodynamic
response of a model M dwarf atmosphere to a beam of nonthermal
electrons. Radiative back-warming through numerous soft X-ray,
extreme-ultraviolet, and ultraviolet transitions are also included. The
equations of radiative transfer and statistical equilibrium are treated
in non-LTE for many transitions of hydrogen, helium, and the Ca II
ion, allowing the calculation of detailed line profiles and continuum
radiation. Two simulations were carried out, with electron beam fluxes
corresponding to moderate and strong beam heating. In both cases we
find that the dynamics can be naturally divided into two phases: an
initial gentle phase in which hydrogen and helium radiate away much
of the beam energy and an explosive phase characterized by large
hydrodynamic waves. During the initial phase, lower chromospheric
material is evaporated into higher regions of the atmosphere, causing
many lines and continua to brighten dramatically. The He II 304 line
is especially enhanced, becoming the brightest line in the flaring
spectrum. The hydrogen Balmer lines also become much brighter and show
very broad line widths, in agreement with observations. We compare
our predicted Balmer decrements to decrements calculated for several
flare observations and find the predictions to be in general agreement
with the observations. During the explosive phase both condensation and
evaporation waves are produced. The moderate flare simulation predicts
a peak evaporation wave of ~130 km s<SUP>-1</SUP> and a condensation
wave of ~30 km s<SUP>-1</SUP>. The velocity of the condensation wave
matches velocities observed in several transition region lines. The
optical continuum also greatly intensifies, reaching a peak increase
of 130% (at 6000 Å) for the strong flare, but does not match observed
white-light spectra.
---------------------------------------------------------
Title: Radiative Hydrodynamic Models of the Optical and Ultraviolet
Emission from Solar Flares
Authors: Allred, Joel C.; Hawley, Suzanne L.; Abbett, William P.;
Carlsson, Mats
2005ApJ...630..573A Altcode: 2005astro.ph..7335A
We report on radiative hydrodynamic simulations of moderate and strong
solar flares. The flares were simulated by calculating the atmospheric
response to a beam of nonthermal electrons injected at the apex of a
one-dimensional closed coronal loop and include heating from thermal
soft X-ray, extreme ultraviolet, and ultraviolet (XEUV) emission. The
equations of radiative transfer and statistical equilibrium were
treated in non-LTE and solved for numerous transitions of hydrogen,
helium, and Ca II, allowing the calculation of detailed line profiles
and continuum emission. This work improves on previous simulations
by incorporating more realistic nonthermal electron beam models and
includes a more rigorous model of thermal XEUV heating. We find that
XEUV back-warming contributes less than 10% of the heating, even in
strong flares. The simulations show elevated coronal and transition
region densities resulting in dramatic increases in line and continuum
emission in both the UV and optical regions. The optical continuum
reaches a peak increase of several percent, which is consistent with
enhancements observed in solar white-light flares. For a moderate flare
(~M class), the dynamics are characterized by a long gentle phase of
near balance between flare heating and radiative cooling, followed
by an explosive phase with beam heating dominating over cooling and
characterized by strong hydrodynamic waves. For a strong flare (~X
class), the gentle phase is much shorter, and we speculate that for even
stronger flares the gentle phase may be essentially nonexistent. During
the explosive phase, synthetic profiles for lines formed in the upper
chromosphere and transition region show blueshifts corresponding to
a plasma velocity of ~120 km s<SUP>-1</SUP>, and lines formed in the
lower chromosphere show redshifts of ~40 km s<SUP>-1</SUP>.
---------------------------------------------------------
Title: Convective Dynamos and the Minimum X-Ray Flux in Main-Sequence
Stars
Authors: Bercik, D. J.; Fisher, G. H.; Johns-Krull, Christopher M.;
Abbett, W. P.
2005ApJ...631..529B Altcode: 2005astro.ph..6027B
The objective of this paper is to investigate whether a convective
dynamo can account quantitatively for the observed lower limit of X-ray
surface flux in solar-type main-sequence stars. Our approach is to use
three-dimensional numerical simulations of a turbulent dynamo driven
by convection to characterize the dynamic behavior, magnetic field
strengths, and filling factors in a nonrotating stratified medium and
to predict these magnetic properties at the surface of cool stars. We
use simple applications of stellar structure theory for the convective
envelopes of main-sequence stars to scale our simulations to the
outer layers of stars in the F0-M0 spectral range, which allows us
to estimate the unsigned magnetic flux on the surface of nonrotating
reference stars. We find agreement between our G0 star calculations
and the observed unsigned magnetic flux density in the quiet Sun. With
these magnetic flux estimates we use the recent results of Pevtsov
et al. to predict the level of X-ray emission from such a turbulent
dynamo and find that our results compare well with observed lower
limits of surface X-ray flux. If we scale our predicted X-ray fluxes
to Mg II fluxes, we also find good agreement with the observed lower
limit of chromospheric emission in K dwarfs. This suggests that dynamo
action from a convecting, nonrotating plasma is a viable alternative to
acoustic heating models as an explanation for the basal emission level
seen in chromospheric, transition-region, and coronal diagnostics from
late-type stars.
---------------------------------------------------------
Title: Turbulent Dynamos and the Minimum X-ray Flux in Solar-Type
Main Sequence Stars
Authors: Bercik, D. J.; Fisher, G. H.; Johns-Krull, C. M.; Abbett,
W. P.
2005AGUSMSP43B..01B Altcode:
We investigate whether a small-scale turbulent dynamo can account
quantitatively for the observed lower limit of X-ray surface flux in
solar-type main sequence stars. Our approach is to use 3D numerical
simulations of a turbulent dynamo driven by convection to characterize
the dynamic behavior, magnetic field strengths, and filling factors in a
non-rotating stratified medium, and to predict these magnetic properties
at the surface of cool stars. We use simple applications of stellar
structure theory for the convective envelopes of main-sequence stars
to scale our simulations to the outer layers of stars in the F0--M0
spectral range, which allows us to estimate the unsigned magnetic flux
on the surface of non-rotating reference stars. With these estimates
we use the observed magnetic flux--X-ray flux correlation of Pevtsov et
al. (2003) to predict the level of X-ray emission from such a turbulent
dynamo, and find that our results compare well with observed lower
limits of surface X-ray flux. This suggests that dynamo action from a
convecting, non-rotating plasma is a viable alternative to acoustic
heating models as an explanation for the basal emission level seen
in chromospheric, transition region, and coronal diagnostics from
late-type stars.
---------------------------------------------------------
Title: 3D MHD Simulations of Magnetic Flux Emergence in Active Regions
Authors: Abbett, W. P.
2005AGUSMSP41A..11A Altcode:
We report on the progress of 3D simulations of active region magnetic
flux emergence (and decay) through the stratified, sub-photospheric
layers of the upper convection zone into the solar atmosphere and low
corona. We use a recently-developed 3D semi-implicit MHD code (with a
non-uniform, adaptive mesh) to address the inherent stiffness of the
system of equations, and will compare our results with similar studies
using second-order accurate, fully explicit numerical schemes.
---------------------------------------------------------
Title: The photospheric boundary of Sun-to-Earth coupled models
Authors: Abbett, W. P.; Mikić, Z.; Linker, J. A.; McTiernan, J. M.;
Magara, T.; Fisher, G. H.
2004JASTP..66.1257A Altcode: 2004JATP...66.1257A
The least understood component of the Sun-to-Earth coupled system
is the solar atmosphere—the visible layers of the Sun that
encompass the photosphere, chromosphere, transition region and
low corona. Coronal mass ejections (CMEs), principal drivers of
space weather, are magnetically driven phenomena that are thought to
originate in the low solar corona. Their initiation mechanism, however,
is still a topic of great debate. If we are to develop physics-based
models with true predictive capability, we must progress beyond
simulations of highly idealized magnetic configurations, and develop
the techniques necessary to incorporate observations of the vector
magnetic field at the solar photosphere into numerical models of the
solar corona. As a first step toward this goal, we drive the SAIC
coronal model with the complex magnetic fields and flows that result
from a sub-photospheric MHD simulation of an emerging active region. In
particular, we successfully emerge a twisted Ω-loop into a pre-existing
coronal arcade. <P />To date, it is not possible to directly measure
the magnetic field in the solar corona. Instead, we must rely on
non-potential extrapolations to generate the twisted, pre-eruptive
coronal topologies necessary to initiate data-driven MHD simulations
of CMEs. We therefore investigate whether a non-constant-α force-free
extrapolation can successfully reproduce the magnetic features of a
self-consistent MHD simulation of flux emergence through a stratified
model atmosphere. We generate force-free equilibria from simulated
photospheric and chromospheric vector magnetograms, and compare these
results to the MHD calculation. We then apply these techniques to an IVM
(Mees Solar Observatory) vector magnetogram of NOAA active-region 8210,
a source of a number of eruptive events on the Sun.
---------------------------------------------------------
Title: The Dynamic Evolution of Twisted Magnetic Flux Tubes in a
Three-dimensional Convecting Flow. II. Turbulent Pumping and the
Cohesion of Ω-Loops
Authors: Abbett, W. P.; Fisher, G. H.; Fan, Y.; Bercik, D. J.
2004ApJ...612..557A Altcode:
We present a set of three-dimensional MHD simulations using the
anelastic approximation of active region-scale flux ropes embedded
in a turbulent, stratified model convection zone. We simulate the
evolution of Ω-loops and other magnetic structures of varying field
strengths, helicities, and morphologies in both rotating and nonrotating
background states. We show that if the magnetic energy of a flux tube
is weak relative to the kinetic energy density of strong downdrafts,
convective flows dominate the evolution, flux tubes of any shape
rapidly lose cohesion, and the magnetic field redistributes itself
throughout the domain over timescales characteristic of convective
turnover. We determine the conditions under which magnetic tension
resulting from field line twist can provide the force necessary to
prevent a relatively weak flux tube from losing cohesion during its
ascent through the turbulent convection zone. Our simulations show
that there is no initial tendency for a horizontal magnetic flux tube
or layer to be preferentially transported in one vertical direction
over the other solely as a result of the presence of an asymmetric
vertical flow field. However, as the simulations progress, there is
a transient net transport of magnetic flux into the lower half of the
computational domain as the distribution of the magnetic field changes
and flux is expelled from cell centers into converging downflows and
intergranular lanes. This pumping mechanism is weak and uncorrelated
with the degree of vertical flow asymmetry. We find that the strong
turbulent pumping evident in simulations of penetrative convection-the
efficient transport of magnetic flux to the base of the convection
zone over several local turnover times-does not manifest itself in a
closed domain in the absence of a convective overshoot layer. Thus,
we suggest that this rapid redistribution of flux is primarily due to
the penetration of magnetic flux into the stable layer where it remains
over a timescale that far exceeds that of convective turnover. We also
find that different treatments of the viscosity of a Newtonian fluid-in
which the coefficient of either kinematic or dynamic viscosity is
held constant throughout the domain-do not affect the global average
evolution of embedded magnetic structures, although the details of
the evolution may differ between models.
---------------------------------------------------------
Title: ILCT: Recovering Photospheric Velocities from Magnetograms
by Combining the Induction Equation with Local Correlation Tracking
Authors: Welsch, B. T.; Fisher, G. H.; Abbett, W. P.; Regnier, S.
2004ApJ...610.1148W Altcode:
We present three methods for deriving the velocity field in magnetized
regions of the Sun's photosphere. As a preliminary step, we introduce
a Fourier-based local correlation tracking (LCT) routine that we term
“FLCT.” By explicitly employing the observation made by Démoulin
& Berger, that results determined by LCT applied to magnetograms
involve a combination of all components of the velocity and magnetic
fields, we show that a three-component velocity field can be derived, in
a method we term algebraic decomposition, or ADC. Finally, we introduce
ILCT, a method that enforces consistency between the normal component
of the induction equation and results obtained from LCT. When used with
photospheric vector magnetograms, ILCT determines a three-component
photospheric velocity field suitable for use with time sequences of
such magnetograms to drive boundary conditions for MHD simulations
of the solar corona. We present results from these methods applied to
vector magnetograms of NOAA AR 8210 on 1998 May 1.
---------------------------------------------------------
Title: Radiative Hydrodynamic Simulations of Solar and Stellar Flares
Authors: Allred, J. C.; Hawley, S. L.; Abbett, W. P.
2004AAS...204.0305A Altcode: 2004BAAS...36..671A
We have constructed radiative hydrodynamic simulations of the effects
of flare heating on model solar and dMe stellar atmospheres. The
heating is assumed to be driven by a beam of non-thermal electrons
originating in the corona and impacting on the lower transition region
and chromosphere. We use thick target bremsstrahlung fits to RHESSI
hard X-ray observations of the July 23, 2002 and February 26, 2002
flares to model the electron beam. Our simulations include detailed
calculations of numerous bound-bound and bound-free transitions which
we compare with line profiles measured during flares on the Sun and on
the dMe star AD Leo. We also investigate the possibility that the 511
keV emission line is produced from a significant amount of material
at transition region temperatures.
---------------------------------------------------------
Title: ILCT: Combining Local Correlation Tracking with the Magnetic
Induction Equation
Authors: Fisher, G. H.; Welsch, B. T.; Abbett, W. P.; Regnier, S.
2004AAS...204.8805F Altcode: 2004BAAS...36..820F
In order to use sequences of vector magnetogram data as input to MHD
simulations of the solar atmosphere, one must ensure that the data is
consistent with the MHD induction equation. We describe a new technique,
ILCT, that uses local correlation tracking to determine a 3-D flow
field that is consistent with the ideal MHD induction equation. The
flow fields are thus suitable for incorporation into the photospheric
boundary of an MHD simulation of the solar atmosphere.
---------------------------------------------------------
Title: HST, EUVE and Ground-Based Observations of Flares on AD Leo
Authors: Allred, J. C.; Hawley, S. L.; Johns-Krull, C. M.; Fisher,
G. H.; Abbett, W. P.; Avgoloupis, S. I.; Seiradakis, J. H.
2004IAUS..219..829A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Radiative Hydrodynamic Models of Solar White Light Flares
Authors: Allred, J. C.; Hawley, S. L.; Abbett, W. P.; Fisher, G. H.;
Hudson, H. S.; Metcalf, T. R.
2003AGUFMSH22A0175A Altcode:
We report on theoretical radiative hydrodynamic simulations of solar
white light flares. The solar atmosphere is modeled in detail from
the transition region to the photosphere. The coronal pressure and
X-ray backheating are included self-consistently. Flare heating is
assumed to be from an electron beam which is modeled for several
white light flares using data from RHESSI, TRACE and Yohkoh. We also
investigate the possibility that the 511 keV line width is produced
from a significant column depth of atmosphere at transition region
temperatures. We compare our new solar flare models to previous results,
and to models of M dwarf stellar flares.
---------------------------------------------------------
Title: Temperature, Density, and Magnetic Field Reconstructions of
Active Region Coronae
Authors: Lundquist, L. L.; Fisher, G. H.; Régnier, S.; Liu, Y.;
Abbett, W. P.
2003AGUFMSH42B0509L Altcode:
We present simulated coronal emission pictures of some case-study solar
active regions, including NOAA-designated regions 8210 and 8038. The
simulated emissions are calculated from a 3-d temperature, density,
and magnetic field model of the corona based on first principles. The
method involves a static energy balance along individual coronal loops,
with the heating term taken from a given coronal heating theory. The
predicted emissions can be compared with observed X-ray and UV satellite
images. By comparing the predictions of various heating theories with
observations, we can determine constraints on the probable mechanisms
of coronal heating. The model is also useful for a variety of other
applications, such as testing of coronal magnetic field extrapolation
techniques, calculations of wave propagation and shock phenomena, and
testing assumptions about the spatial distribution of heating along
loops. This work was supported by a DoD/AFOSR MURI grant, "Understanding
Magnetic Eruptions and their Interplanetary Consequences."
---------------------------------------------------------
Title: I+LCT: A Method for Determining Photospheric Flows from
Magnetograms
Authors: Welsch, B. T.; Fisher, G. H.; Abbett, W. P.
2003AGUFMSH22A0177W Altcode:
Coronal mass ejections (CME's) are magnetically-driven reconfigurations
of plasma in the low-β solar corona; they are the primary drivers of
space weather. One way to investigate coronal field evolution before
and during such events involves driving MHD simulations of the coronal
magnetic field using data from time series of vector magnetograms. Doing
so requires specification of a three-component velocity field at the
simulated photosphere, consistent with the observed magnetic field
evolution in that layer. Unfortunately, such velocity data are not
generally available. We have developed a method that finds photospheric
plasma velocities consistent with both the flows derived by the familiar
techinique of local correlation tracking (LCT), and the field evolution
described by z-component of the induction equation. We present results
obtained by applying this “I+LCT” technique to vector magnetograms,
and to “false” magnetograms obtained from MHD simulations of
photospheric field evolution.
---------------------------------------------------------
Title: Turbulent Magnetic Field Generation in Rotating Stars
Authors: Bercik, D. J.; Abbett, W. P.; Fisher, G. H.; Fan, Y.
2003AGUFMSH42B0536B Altcode:
Observationally, it has been found that magnetic activity is a
strong function of rotation rate. The connection between rotation and
dynamo-generated fields is not well understood, however. The typical
interface dynamo theory applied to the Sun to describe its activity
cycle assumes the existence of a velocity shear layer. Such a model
is inappropriate for fully convective stars that are nevertheless
active, such as late-type M and L stars and pre-main sequence T Tauri
stars; in these stars a turbulent dynamo is generally believed to
be the mechanism of magnetic field generation. We investigate the
connection between observed activity behavior and magnetic field
generation in fully convective stars through a series of simulations
of the turbulent dynamo. The simulations were performed in a Cartesian
domain using ANMHD, a 3D MHD anelastic code. We compare the resulting
magnetic topologies for a series of Rossby numbers and comment on the
implications for the sizes of coronal loops and activity levels.
---------------------------------------------------------
Title: Incorporating Vector Magnetic Field Measurements into MHD
Models of the Solar Atmosphere
Authors: Abbett, W. P.
2003AGUFMSM11A..03A Altcode:
We report on our efforts to incorporate high cadence vector magnetic
field measurements of the CME and flare producing active region NOAA
8210 (observed from April 28 to May 2 1998) into the photospheric
boundary layers of our 3D MHD models of the solar atmosphere. We
find that it is is essential to be able to specify an initial model
atmosphere that is both consistent with soft X-ray observations of
the corona and with observed vector magnetic field measurements at
the photosphere. Further, MHD codes require that certain components of
the flowfield be specified at the lower boundary in such a way as to
self-consistently update the model photosphere between each successive
magnetogram. We will present the results of our application of several
techniques to infer the velocity field of magnetized plasma in the
photosphere using a time-series of IVM vector magnetograms of NOAA
8210, and will present the results from our latest attempt to model
this complex active region.
---------------------------------------------------------
Title: Multiwavelength Observations of Flares on AD Leonis
Authors: Hawley, Suzanne L.; Allred, Joel C.; Johns-Krull, Christopher
M.; Fisher, George H.; Abbett, William P.; Alekseev, Ilya; Avgoloupis,
Stavros I.; Deustua, Susana E.; Gunn, Alastair; Seiradakis, John H.;
Sirk, Martin M.; Valenti, Jeff A.
2003ApJ...597..535H Altcode:
We report results from a multiwavelength observing campaign conducted
during 2000 March on the flare star AD Leo. Simultaneous data were
obtained from several ground- and space-based observatories, including
observations of eight sizable flares. We discuss the correlation of
line and continuum emission in the optical and ultraviolet wavelength
regimes, as well as the flare energy budget, and we find that the
emission properties are remarkably similar even for flares of very
different evolutionary morphology. This suggests a common heating
mechanism and atmospheric structure that are independent of the detailed
evolution of individual flares. We also discuss the Neupert effect,
chromospheric line broadening, and velocity fields observed in several
transition region emission lines. The latter show significant downflows
during and shortly after the flare impulsive phase. Our observations are
broadly consistent with the solar model of chromospheric evaporation
and condensation following impulsive heating by a flux of nonthermal
electrons. These data place strong constraints on the next generation
of radiative hydrodynamic models of stellar flares.
---------------------------------------------------------
Title: The March 2000 AD Leo Flare Campaign
Authors: Hawley, S. L.; Johns-Krull, C. M.; Fisher, G. H.; Abbett,
W. P.; Seiradakis, J. H.; Avgoloupis, S. I.
2003csss...12..975H Altcode:
Flares are by their nature random and unpredictable events and flare
observations are often the serendipitous result of programs designed for
other scientific endeavors. Thus, few observations of flares covering
multiple wavelength regimes, with both spectroscopic and photometric
information, are available to test stellar flare models. Occasionally,
a bold and reckless team will put together a flare campaign, employing
suitable statistical arguments to convince the relevant telescope
allocation committees that such a campaign will prove fruitful, while
hoping desperately for the combination of clear weather, working
instruments and cooperative star necessary to warrant the herculean
organizational effort. We report here on one such campaign, conducted
during March, 2000 on the dM3e flare star AD Leo.
---------------------------------------------------------
Title: A Magnetohydrodynamic Test of the Wang-Sheeley Model
Authors: Ledvina, S. A.; Luhmann, J. G.; Abbett, W. P.
2003AIPC..679..323L Altcode:
The Wang-Sheeley relationship relates the solar wind speed at the
Earth to the divergence rate of open magnetic flux tubes in the solar
corona. This relationship is based on a statistically significant
correlation between the flux tube divergence parameter “fs” derived
from a photospheric field-based potential field source surface model,
and satellite observations of the solar wind speed. The fast solar wind
emanates from regions of small magnetic divergence, while slow solar
wind comes from regions of high magnetic divergence. Arge and Pizzo
[2] improved the reliability of the method by relating the coronal
flux tube expansion factor to the solar wind speed at the source
surface instead of the satellite. We use a three-dimensional MHD model
of the solar corona to further investigate the implications of the
Wang-Sheeley relationship for solar wind acceleration. The results
suggest what additional heating and momentum inputs may be necessary
in an MHD model to obtain the observed relationship between flux tube
divergence and solar wind speed.
---------------------------------------------------------
Title: A Temperature and Density Model of the Solar Corona
Authors: Lundquist, L. L.; Regnier, S.; Abbett, W. P.; Fisher, G. H.
2003SPD....34.0404L Altcode: 2003BAAS...35..811L
We have developed the foundations of a 3-d global temperature and
density model of the solar corona based on first principles. The method
involves a static energy balance along individual coronal loops, with
the heating term taken from a given coronal heating theory. We use
the model to create synthetic emission images of active regions for
comparison with observed X-ray and UV satellite images. The technique
will enable us to perform a statistical study of active region heating
with Yohkoh data from the last decade, comparing observations with the
predicted emission measures and X-ray morphologies for different heating
theories. The model is also useful for a variety of other applications,
such as calculations of wave propagation and shock phenomena, testing of
coronal magnetic field extrapolation techniques such as the potential
and FFF models, and testing assumptions about the spatial distribution
of heating along loops. <P />We have applied the technique to two
cases: a simulated emerged active region, and NOAA active region
8210. These cases employ a heating term derived from the empirical
relationship of Pevtsov et al. (2003) relating soft X-ray luminosity
to total unsigned magnetic flux for a wide range of solar and stellar
magnetic features. We present results from these two cases, including a
comparison of the synthetic emission images of AR 8210 with Yohkoh SXT
data. This work was supported by a DoD/AFOSR MURI grant, "Understanding
Magnetic Eruptions and their Interplanetary Consequences."
---------------------------------------------------------
Title: Can Simulations of Active Region Magnetic Fields Lead to a
Simplified Model of Turbulent Pumping?
Authors: Abbett, W. P.; Fisher, G. H.; Fan, Y.; Bercik, D. J.
2003SPD....34.1905A Altcode: 2003BAAS...35..842A
We present a series of 3-D MHD simulations in the anelastic
approximation of active region scale magnetic flux ropes embedded in
a highly stratified, turbulent model convection zone. The numerical
calculations are carried out over long time scales (of order a solar
rotation time) at high magnetic Reynolds numbers and suggest that
the process of “turbulent pumping” --- the tendency for magnetic
flux to be efficiently transported from surface layers to the base of
the convection zone --- does not manifest itself in the absence of
a convective overshoot layer. If the overshoot layer is present, we
suggest a simple, statistical model (similar to a 1-D, depth-dependent
eddy-diffusivity treatment with a characteristic time-scale of
supergranulation) that describes the average properties of the flux
storage process.
---------------------------------------------------------
Title: Comparison of the Coronal Magnetic Field Derived from PFSS
and MHD Models
Authors: Ledvina, S. A.; Luhmann, J. G.; Li, Y.; Abbett, W. P.
2003SPD....34.0601L Altcode: 2003BAAS...35..817L
The coronal magnetic field determines many properties of the solar
corona such as the location of the heliospheric current sheet and
regions of high and low speed solar wind. Thus understanding the
structure of the coronal magnetic field is crucial to the understanding
of space weather. Several models use a synoptic map to derive the
structure of the coronal field out to several solar radii. One such
model is the potential field source surface model (PFSS). This model
neglects electric currents between the photosphere and a "source
surface" (typically 2.5 Rs). At the source surface the field lines
are forced to be radial in order to mimic the effects of the solar
wind. In contrast MHD models try to self-consistently derive the coronal
field and the plasma properties of the corona. We compare the coronal
magnetic field structures derived by the PFSS and MHD models in order to
understand what role dynamical effects may have on the field structure.
---------------------------------------------------------
Title: Modeling of the Turbulent Dynamo
Authors: Bercik, D. J.; Fisher, G. H.; Abbett, W. P.
2003SPD....34.1904B Altcode: 2003BAAS...35R.842B
We report on the development progress of a 3-D spherical anelastic
MHD code (SANMHD) that will be used to study the turbulent dynamo in
solar and stellar interiors. SANMHD is a complement to the existing
Cartesian anelastic MHD code, ANMHD; to model the interaction between
penetrative turbulent convection and magnetic field deep in the interior
of stars, it becomes necessary to consider a gravitationally stratified
atmosphere in a spherical geometry. We discuss issues regarding the
creation of a highly modular code that is portable across multiple
platforms, code structuring to allow the flexibility to investigate
a variety of physical scenarios and testing strategies.
---------------------------------------------------------
Title: The Dynamic Evolution of Twisted Magnetic Flux Tubes in a
Three-dimensional Convecting Flow. I. Uniformly Buoyant Horizontal
Tubes
Authors: Fan, Y.; Abbett, W. P.; Fisher, G. H.
2003ApJ...582.1206F Altcode:
We present three-dimensional numerical simulations of the dynamic
evolution of uniformly buoyant, twisted horizontal magnetic flux
tubes in a three-dimensional stratified convective velocity field. Our
calculations are relevant to understanding how stratified convection in
the deep solar convection zone may affect the rise and the structure
of buoyant flux tubes that are responsible for the emergence of solar
active regions. We find that in order for the magnetic buoyancy force
of the tube to dominate the hydrodynamic force due to the convective
downflows, the field strength B of the flux tube needs to be greater
than (H<SUB>p</SUB>/a)<SUP>1/2</SUP>B<SUB>eq</SUB>~3B<SUB>eq</SUB>,
where H<SUB>p</SUB> is the pressure scale height, a is the tube
radius, and B<SUB>eq</SUB> is the field strength in equipartition
with the kinetic energy density of the strong downdrafts. For tubes
of equipartition field strength (B=B<SUB>eq</SUB>), the dynamic
evolution depends sensitively on the local condition of the convective
flow. Sections of the tube in the paths of strong downdrafts are
pinned down to the bottom despite their buoyancy, while the rise
speed of sections within upflow regions is significantly boosted;
Ω-shaped emerging tubes can form between downdrafts. Although flux
tubes with B=B<SUB>eq</SUB> are found to be severely distorted by
convection, the degree of distortion obtained from our simulations
is not severe enough to clearly rule out the Ω-tubes that are able
to emerge between downdrafts as possible progenitors of solar active
regions. As the initial field strength of the tube becomes higher than
the critical value of ~(H<SUB>p</SUB>/a)<SUP>1/2</SUP>B<SUB>eq</SUB>
given above, the dynamic evolution converges toward the results of
previous simulations of the buoyant rise of magnetic flux tubes in a
static, adiabatically stratified model solar convection zone. Tubes
with 10 times the equipartition field strength are found to rise
unimpeded by the downdrafts and are not significantly distorted by
the three-dimensional convective flow.
---------------------------------------------------------
Title: A Coupled Model for the Emergence of Active Region Magnetic
Flux into the Solar Corona
Authors: Abbett, W. P.; Fisher, G. H.
2003ApJ...582..475A Altcode:
We present a set of numerical simulations that model the emergence
of active region magnetic flux into an initially field-free model
corona. We simulate the buoyant rise of twisted magnetic flux tubes
initially positioned near the base of a stable stratified model
convection zone and use the results of these calculations to drive a
three-dimensional magnetohydrodynamic model corona. The simulations show
that time-dependent subsurface flows are an important component of the
dynamic evolution and subsequent morphology of an emerging magnetic
structure. During the initial stages of the flux emergence process,
the overlying magnetic field differs significantly from a force-free
state. However, as the runs progress and boundary flows adjust, most
of the coronal field-with the exception of those structures located
relatively close to the model photosphere-relaxes to a more force-free
configuration. Potential field extrapolations do not adequately
represent the magnetic structure when emerging active region fields
are twisted. In the dynamic models, if arched flux ropes emerge with
nonzero helicity, the overlying field readily forms sigmoid-shaped
structures. However, the chirality of the sigmoid and other details of
its structure depend on the observer's vantage point and the location
within a given loop of emitting plasma. Thus, sigmoids may be an
unreliable signature of the sign and magnitude of magnetic twist.
---------------------------------------------------------
Title: Comparison of the Coronal Magnetic Field Derived from PFSS
and MHD Models
Authors: Ledvina, S. A.; Luhmann, J. G.; Li, Y.; Abbett, W. P.
2002AGUFMSH52A0456L Altcode:
The coronal magnetic field determines many properties of the solar
corona such as the location of the heliospheric current sheet and
regions of high and low speed solar wind. Thus understanding the
structure of the coronal magnetic field is crucial to the understanding
of space weather. Several models use a synoptic map to derive the
structure of the coronal field out to several solar radii. One such
model is the potential field source surface model (PFSS). This model
neglects electric currents between the photosphere and a "source
surface" (typically 2.5 Rs). At the source surface the field lines
are forced to be radial in order to mimic the effects of the solar
wind. In contrast MHD models try to self-consistently derive the coronal
field and the plasma properties of the corona. We compare the coronal
magnetic field structures derived by the PFSS and MHD models in order to
understand what role dynamical effects may have on the field structure.
---------------------------------------------------------
Title: The Dynamic Evolution of Twisted Omega-loops in a 3-D
Convecting Flow
Authors: Abbett, W. P.; Fan, Y.; Fisher, G. H.
2002AGUFMSH52A0474A Altcode:
We present the latest results from 3D MHD simulations (in the anelastic
approximation) of buoyant magnetic flux tubes interacting with turbulent
convection in the solar interior. We focus our study on active region
scale flux ropes and Omega-loops, and perform a large parameter space
study of the effects of not only initial field strength, but twist and
loop geometry on the morphology and dynamics of sub-surface magnetic
structures. We also investigate the effects of different numerical
treatments of viscosity, and quantify the amount of magnetic field in
each simulation that succumbs to the effects of turbulent pumping.
---------------------------------------------------------
Title: The Rise of Twisted Horizontal Flux Tubes in a 3D Convecting
Flow
Authors: Fan, Y.; Abbett, W. P.; Fisher, G. H.
2002AAS...200.0306F Altcode: 2002BAAS...34..642F
We present 3D numerical simulations of the dynamic evolution of twisted
horizontal magnetic flux tubes in a stratified convecting convection
zone. We investigate how the trajectory, rise velocity, and cohesion of
the buoyant flux tubes are affected by the 3D stratified convection. It
is found that the field strength of the magnetic flux tube needs to
be significantly above the value of equipartition with the kinetic
energy of convection in order for the flux tube to rise cohesively
to the top of the stratified domain. These simulations add further
support to the strong toroidal field strength ( ~ 5 x 10<SUP>4</SUP> G
to 10<SUP>5</SUP> G) at the base of the solar convection zone, suggested
by previous thin flux tube calculations of emerging flux tubes through
the solar convective envelope. NCAR is sponsored by the National Science
Foundation. Part of this work was carried out while the authors were
participating in the solar magnetic field program held at ITP, UCSB.
---------------------------------------------------------
Title: A Magnetohydrodynamic Test of the Wang-Sheeley Model
Authors: Ledvina, S. A.; Abbett, W. P.; Luhmann, J. G.
2002AAS...200.5714L Altcode: 2002BAAS...34Q.739L
The Wang-Sheeley empirical model enables the calculation of the
solar wind speed at Earth from the divergence of magnetic flux
tubes in a synoptic map-based potential field source surface model
of the coronal magnetic field. The formula used for this purpose was
derived from observations of the solar wind speed at 1 AU. According
to the model, fast solar wind emanates from regions of small magnetic
field divergence, while slow solar wind comes from high divergence
regions. Arge and Pizzo (2000) recently improved the Wang-Sheeley
formula by taking the stream interaction effects between the Sun
and 1 AU into account. We use a three-dimensional MHD model of the
solar corona to test the assumptions of the Wang- Sheeley model
and the improvements made by Arge and Pizzo for various magnetic
configurations. These results provide insight into what additional
coronal heating may be implied for different flux tube geometries in
order to obtain the observed relationship with solar wind speed.
---------------------------------------------------------
Title: Numerical Simulations of Magnetic Flux Emergence in Active
Regions
Authors: Abbett, W. P.
2002AAS...200.7906A Altcode: 2002BAAS...34..780A
Understanding the sub-photospheric structure and dynamics of emerging
active region magnetic fields, and how these fields are coupled to
structures observed above the photosphere, is important to a variety of
ongoing research projects in both the solar physics and space science
communities (for example, the effort to predict the onset of intense
episodes of solar activity such as CMEs and flares). Over the past
decade, much progress has been made by using 2-D MHD codes and the 1-D
“thin flux tube” approximation to describe the evolution of buoyant
magnetic flux tubes in the solar interior. However, in recent years,
the rapid evolution of computer technology, coupled with advances
in computational algorithms, have made it possible to use physically
self-consistent, 3-D MHD numerical simulations to model the evolution
of strong magnetic fields through stratified model convection zones
without the restrictive assumptions of earlier models. This review will
summarize efforts to use modern 3-D codes as tools to test predictions
of earlier theoretical models and to interpret observational data. The
emphasis will be on the progress made in modeling emerging magnetic flux
in the solar interior; however, a brief overview of recent efforts to
couple sub-photospheric simulations to models of the solar atmosphere
and corona will also be presented.
---------------------------------------------------------
Title: New coupled models of emerging magnetic flux in active regions
Authors: Abbett, W.; Ledvina, S.; Fisher, G.
2002ocnd.confE..22A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: How do emerging magnetic fields affect the solar coronal
field configuration?
Authors: LI, Y.; Luhmann, J. G.; Abbett, W.; Linker, J.; Lionello,
R.; Mikic, Z.
2001AGUFMSH11C0719L Altcode:
Experiments are carried out to study the coronal field response to an
emerging active region into a simple background global magnetic field
using potential field source surface models. The emerging active region
used is the radial component of the magnetic field of an emerging flux
rope from an ANMHD simulation. When the active region is emerging
into a dipole field, it introduces polar coronal hole extensions,
warps the source surface neutral lines, and changes the field line
connections. The active region internal field line connections are also
changed to be different from an isolated active region. The relative
strength of the background and active region affect the extent of the
changes that occur. The field distribution of the background global
field is important, and different background with the same emerging
active region may result in different coronal features. A few examples
of different background fields with the emerging active region will be
presented and compared. A global MHD simulation is also in preparation
using the same global magnetic field with the emerging active region
as the boundary condition.
---------------------------------------------------------
Title: A Magnetohydrodynamic Test of the Wang-Sheeley Model
Authors: Ledvina, S. A.; Abbett, W. P.; Li, Y.; Luhmann, J. G.
2001AGUFMSH31A0695L Altcode:
The Wang-Sheeley empirical model relates the solar wind speed observed
at the Earth with the divergence rate of magnetic flux tubes expanding
in the solar corona. This model is based on a statistically significant
correlation between an open flux tube divergence parameter "fs" derived
from photospheric field synoptic maps, and satellite observations of
the solar wind speed. They found that the fast solar wind emanates
from regions of small magnetic divergence, while slow solar wind comes
from regions of high magnetic divergence. Arge and Pizzo (2000) have
since improved the reliability of the Wang-Sheeley model by including
an empirical function that relates the magnetic expansion factor to
the solar wind speed at the source surface, and a scheme to account
for stream interactions as the solar wind propagates outward. We use
a three-dimensional MHD model of the solar corona to empirically
test the Wang-Sheely model and the improvements made by Arge and
Pizzo. These results may provide insight into what additional heating
may be necessary in different flux tube geometries in order to obtain
to the observed relationship with solar
---------------------------------------------------------
Title: Flux-loss of buoyant ropes interacting with convective flows
Authors: Dorch, S. B. F.; Gudiksen, B. V.; Abbett, W. P.; Nordlund, Å.
2001A&A...380..734D Altcode: 2001astro.ph.10205D
We present 3-d numerical magneto-hydrodynamic simulations of a buoyant,
twisted magnetic flux rope embedded in a stratified, solar-like model
convection zone. The flux rope is given an initial twist such that it
neither kinks nor fragments during its ascent. Moreover, its magnetic
energy content with respect to convection is chosen so that the flux
rope retains its basic geometry while being deflected from a purely
vertical ascent by convective flows. The simulations show that magnetic
flux is advected away from the core of the flux rope as it interacts
with the convection. The results thus support the idea that the amount
of toroidal flux stored at or near the bottom of the solar convection
zone may currently be underestimated.
---------------------------------------------------------
Title: New Coupled Models of Magnetic Flux in Active Regions
Authors: Abbett, W. P.; Ledvina, S. A.; Fisher, G. H.; MacNeice, P.
2001AGUFMSH11C0727A Altcode:
We report progress in our efforts to use the publicly available domain
decomposition and adaptive mesh refinement framework “PARAMESH” to
couple our 3D anelastic MHD (ANMHD) model of active region magnetic
flux in the solar convection zone with a simple, fully compressible
ZEUS3D MHD model of the photosphere, transition region, and low corona.
---------------------------------------------------------
Title: 3-D MHD Simulations of Flux Tube Emergence
Authors: Abbett, W. P.; Fisher, G. H.
2001AGUSM..SH41A10A Altcode:
We present initial results from coupled 3-D MHD simulations of twisted
magnetic flux tubes that have risen through a model solar convection
zone and emerged into the lower corona. We use the anelastic code
“ANMHD” to simulate the rise of buoyant flux tubes through the
solar convection zone, and use this data to generate a photospheric
boundary that drives a simple simulation of the solar transition region
and corona using a modified version of the publicly available code
“ZEUS3D”
---------------------------------------------------------
Title: Initial Behavior of a Buoyant Magnetic Flux Tube Imbedded in
a Rotating Medium
Authors: Fisher, G. H.; Abbett, W. P.
2001AGUSM..SP51B11F Altcode:
In a non-rotating medium with gravity, an initially stationary, buoyant,
untwisted magnetic flux tube will generate 2 counter-rotating vortices
as it begins rising. In a 2-D geometry, these vortices ultimately split
the flux tube into two fragments, which then repel one another. The
trajectory of the flux tube fragments can be predicted extremely
well by using a simple analytical treatment based on the initial
behavior of the flux tube (see e.g. Longcope, Fisher, and Arendt 1996,
Ap.J. 464, 999). Numerical simulations in both 2-D and 3-D geometries
show that rotation dramatically changes this behavior, acting to
strongly suppress magnetic flux tube fragmentation (see e.g. Wissink et
al. 2000, Ap.J. 536, 982 and Abbett, Fisher, & Fan 2001, Ap. J. 546,
1194). Coriolis forces deflect the motions that otherwise would result
in strong circulation around the flux tube fragments. In the same
spirit as the analytical treatment of Longcope, Fisher & Arendt,
we derive equations that describe the initial flow pattern for a 2-D
buoyant, untwisted magnetic flux tube rising in a rotating medium,
and compare these results to those from numerical simulations.
---------------------------------------------------------
Title: The Emergence of Magnetic Flux in Active Regions
Authors: Abbett, W. P.; Fisher, G. H.; Fan, Y.
2001IAUS..203..225A Altcode:
Over the past decade, “thin flux tube” models have proven successful
in explaining many properties of active regions in terms of magnetic
flux tube dynamics in the solar interior. Unfortunately, recent,
more sophisticated two-dimensional MHD simulations of the emergence
of magnetic flux have shown that many of the assumptions adopted in
the thin flux tube approximation are invalid. For example, unless
the flux tubes exhibit a large amount of initial field line twist ---
and observations of emerging active regions suggest they do not ---
they will fragment (break apart) before they are able to emerge through
the surface. We attempt to resolve this paradox using a number of 3-D
MHD simulations (in the anelastic approximation) that describe the
rise and fragmentation of twisted magnetic flux tubes. We find that
the degree of fragmentation of an evolving Omega-loop depends strongly
on the three-dimensional geometry of the tube --- the greater the apex
curvature, the lesser the degree of fragmentation for a fixed amount of
initial twist. We also find that the Coriolis force plays a dynamically
important role in the evolution and emergence of magnetic flux. We are
able to infer general observational characteristics of the emerging
flux, and compare our theoretical data with recent observations.
---------------------------------------------------------
Title: The Effects of Rotation on the Evolution of Rising Omega
Loops in a Stratified Model Convection Zone
Authors: Abbett, W. P.; Fisher, G. H.; Fan, Y.
2001ApJ...546.1194A Altcode: 2000astro.ph..8501A
We present three-dimensional MHD simulations of buoyant magnetic flux
tubes that rise through a stratified model convection zone in the
presence of solar rotation. The equations of MHD are solved in the
anelastic approximation, and the results are used to determine the
effects of solar rotation on the dynamic evolution of an Ω-loop. We
find that the Coriolis force significantly suppresses the degree
of fragmentation at the apex of the loop during its ascent toward
the photosphere. If the initial axial field strength of the tube is
reduced, then, in the absence of forces due to convective motions,
the degree of apex fragmentation is also reduced. Our simulations
confirm the results of thin flux-tube calculations that show the
leading polarity of an emerging active region positioned closer to
the equator than the trailing polarity and the trailing leg of the
loop oriented more vertically than the leading leg. We show that the
Coriolis force slows the rise of the tube and induces a retrograde
flow in both the magnetized and unmagnetized plasma of an emerging
active region. Observationally, we predict that this flow will appear
to originate at the leading polarity and will terminate at the trailing
polarity.
---------------------------------------------------------
Title: Erratum: The Three-dimensional Evolution of Rising, Twisted
Magnetic Flux Tubes in a Gravitationally Stratified Model Convection
Zone
Authors: Abbett, W. P.; Fisher, G. H.; Fan, Y.
2000ApJ...542.1119A Altcode:
In the article “The Three-dimensional Evolution of Rising, Twisted
Magnetic Flux Tubes in a Gravitationally Stratified Model Convection
Zone” by W. P. Abbett, G. H. Fisher, and Y. Fan (ApJ, 540, 548
[2000]), an error was introduced into one of the equations during
the production process. A cross product symbol was mistakenly
removed from equation (2). The corrected equation is as follows:
ρ<SUB>0</SUB>(∂v/∂t+v˙∇v)= -∇p<SUB>1</SUB>+ρ<SUB>1</SUB>g +
1/4π (∇XB)XB+∇˙Π. The Press sincerely apologizes for this error.
---------------------------------------------------------
Title: The Three-dimensional Evolution of Rising, Twisted Magnetic
Flux Tubes in a Gravitationally Stratified Model Convection Zone
Authors: Abbett, W. P.; Fisher, G. H.; Fan, Y.
2000ApJ...540..548A Altcode: 2000astro.ph..4031A
We present three-dimensional numerical simulations of the rise and
fragmentation of twisted, initially horizontal magnetic flux tubes that
evolve into emerging Ω-loops. The flux tubes rise buoyantly through an
adiabatically stratified plasma that represents the solar convection
zone. The MHD equations are solved in the anelastic approximation,
and the results are compared with studies of flux-tube fragmentation
in two dimensions. We find that if the initial amount of field line
twist is below a critical value, the degree of fragmentation at the
apex of a rising Ω-loop depends on its three-dimensional geometry: the
greater the apex curvature of a given Ω-loop, the lesser the degree
of fragmentation of the loop as it approaches the photosphere. Thus,
the amount of initial twist necessary for the loop to retain its
cohesion can be reduced substantially from the two-dimensional
limit. The simulations also suggest that, as a fragmented flux
tube emerges through a relatively quiet portion of the solar disk,
extended crescent-shaped magnetic features of opposite polarity should
form and steadily recede from one another. These features eventually
coalesce after the fragmented portion of the Ω-loop emerges through
the photosphere.
---------------------------------------------------------
Title: 3D MHD Simulation of Flux Tube Dynamics: Comparison with Thin
Flux Tube Models
Authors: Fisher, G. H.; Abbett, W. P.; Fan, Y.
2000SPD....31.0135F Altcode: 2000BAAS...32..807F
We have used the anelastic 3D MHD code “ANMHD” to perform simulations
of emerging magnetic flux tubes moving in a gravitationally stratified
background model representing the solar convection zone. The MHD
model is computed within a local Cartesian geometry, with Coriolis
forces included, using the f-plane approximation. The evolution of
flux tubes computed with the code will be compared and contrasted with
results computed with the thin flux tube approximation. This work was
supported by NASA and NSF.
---------------------------------------------------------
Title: The Cohesion of 3-D Magnetic Flux Tubes in a Rotating,
Stratified Model Convection Zone
Authors: Abbett, W. P.; Fisher, G.; Fan, Y.
2000SPD....31.0136A Altcode: 2000BAAS...32..807A
We present the latest results from a series of 3-D MHD simulations in
the anelastic approximation that describe the rise of magnetic flux
tubes through an adiabatically stratified model convection zone. The
effects of solar rotation and the Coriolis force are included in
the models. The simulations begin with initially horizontal magnetic
flux tubes which subsequently evolve into Omega-loops. We find that
the degree of “fragmentation” at the apex of a rising Omega-loop
depends strongly on both the three-dimensional geometry of the loop,
and on the field strength along the axis of the initial tube. Loops
with a relatively high degree of apex curvature, and of moderate to low
initial axial field strength retain their cohesion throughout their
rise toward the photosphere --- even in the absence of initial field
line twist. We are able to infer general observational characteristics
of the emerging flux, and compare our theoretical data with recent
observations of active regions. This work was funded by NSF grants
AST 98-19727 and ATM 98-96316, and by NASA grant NAGS-8468. The
computations were partially supported by the National Center for
Atmospheric Research, and the National Computational Science Alliance.
---------------------------------------------------------
Title: Magnetic flux tubes inside the sun
Authors: Fisher, G. H.; Fan, Y.; Longcope, D. W.; Linton, M. G.;
Abbett, W. P.
2000PhPl....7.2173F Altcode:
Bipolar magnetic active regions are the largest concentrations of
magnetic flux on the Sun. In this paper, the properties of active
regions are investigated in terms of the dynamics of magnetic flux
tubes which emerge from the base of the solar convection zone, where
the solar cycle dynamo is believed to operate, to the photosphere. Flux
tube dynamics are computed with the “thin flux tube” approximation,
and by using magnetohydrodynamics simulation. Simulations of active
region emergence and evolution, when compared with the known observed
properties of active regions, have yielded the following results: (1)
The magnetic field at the base of the convection zone is confined to
an approximately toroidal geometry with a field strength in the range
3-10×10<SUP>4</SUP> G. The latitude distribution of the toroidal
field at the base of the convection zone is more or less mirrored by
the observed active latitudes; there is not a large poleward drift of
active regions as they emerge. The time scale for emergence of an active
region from the base of the convection zone to the surface is typically
2-4 months. (2) The tilt of active regions is due primarily to the
Coriolis force acting to twist the diverging flows of the rising flux
loops. The dispersion in tilts is caused primarily by the buffeting of
flux tubes by convective motions as they rise through the interior. (3)
Coriolis forces also bend active region flux tube shapes toward the
following (i.e., antirotational) direction, resulting in a steeper
leg on the following side as compared to the leading side of an active
region. When the active region emerges through the photosphere, this
results in a more rapid separation of the leading spots away from the
magnetic neutral line as compared to the following spots. This bending
motion also results in the neutral line being closer to the following
magnetic polarity. (4) The properties of the strongly sheared, flare
productive δ-spot active regions can be accounted for by the dynamics
of highly twisted Ω loops that succumb to the helical kink instability
as they emerge through the solar interior.
---------------------------------------------------------
Title: Dynamic Models of Optical Emission in Impulsive Solar Flares
Authors: Abbett, William P.; Hawley, Suzanne L.
1999ApJ...521..906A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Non-LTE Dynamic Models of Optical Emission During Solar Flares
Authors: Abbett, W. P.
1999AAS...194.2206A Altcode: 1999BAAS...31..860A
The results from a non-LTE radiative-hydrodynamic model of a flare
loop, from its apex in the corona to its footpoints in the photosphere,
are presented. The effects of non-thermal heating of the lower solar
atmosphere by accelerated electrons during the impulsive phase, and the
subsequent effects of soft X-ray irradiation of the chromosphere from
the flare-heated transition region and corona during the beginning of
the gradual phase are investigated. During the impulsive phase, the
models show a significant continuum (or “white light”) brightening
resulting from increased hydrogen recombination radiation in the upper
chromosphere at the point where the accelerated electrons deposit the
bulk of their energy. Additionally, the models produce a measurable
time lag between the brightening of the near wings of H-alpha and the
brightening of the Paschen continuum. This work was funded in part
by NSF grants AST 96-16886 and AST 94-57455. The computations were
partially supported by the National Computational Science Alliance,
and utilized the NCSA SGI/CRAY Power Challenge Array.
---------------------------------------------------------
Title: Dynamical Solar Flare Model Atmospheres
Authors: Abbett, W. P.; Hawley, S. L.
1999ASPC..158..212A Altcode: 1999ssa..conf..212A
No abstract at ADS
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Title: A Theoretical Investigation of Optical Emission in Solar Flares
Authors: Abbett, William Paul
1998PhDT.........1A Altcode: 1998PhDT........91A; 1998PhDT........87A
A dynamic theoretical model of a flare loop from its footpoints in the
photosphere to its apex in the corona is presented, and the effects of
non-thermal heating of the lower atmosphere by accelerated electrons
and soft X-ray irradiation from the flare heated transition region and
corona are investigated. Important transitions of hydrogen, helium,
and singly ionized calcium and magnesium are treated in non-LTE. Three
main conclusions are drawn from the models. First, even the strongest of
impulsive events can be described as having two phases: a gentle phase
characterized by a state of near equilibrium, and an explosive phase
characterized by large material flows, and strong hydrodynamic waves
and shocks. During the gentle phase, one or possibly two temperature
'plateaus' form in the upper chromosphere. The line emission generated
in these regions produces profiles that are generally symmetric and
undistorted, in contrast to emission produced during the explosive
phase, where large velocity gradients that occur in the upper atmosphere
produce line profiles that are highly asymmetric and show large emission
peaks and troughs. Second, a significant continuum (or 'white light')
brightening results from increased hydrogen recombination radiation
in the upper chromosphere at the point where the accelerated electrons
deposit the bulk of their energy. Third, there exists a measurable time
lag between the brightening of the near wings of Hα and the brightening
of the Paschen continuum. This delay is controlled by the amount of time
it takes for electron densities in the upper chromosphere to become
high enough, and the densities of hydrogen atoms in high energy bound
states to become low enough, to allow the number of recombinations to
dominate the number of photoionizations in the region.
---------------------------------------------------------
Title: Non-LTE Radiative-hydrodynamic Models of Solar Flares
Authors: Abbett, William P.; Hawley, Suzanne L.
1997BAAS...29Q1120A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Convection: Comparison of Numerical Simulations and
Mixing-Length Theory
Authors: Abbett, William P.; Beaver, Michelle; Davids, Barry;
Georgobiani, Dali; Rathbun, Pamela; Stein, Robert F.
1997ApJ...480..395A Altcode:
We compare the results of realistic numerical simulations of convection
in the superadiabatic layer near the solar surface with the predictions
of mixing-length theory. We find that the peak values of such quantities
as the temperature gradient, the temperature fluctuations, and the
velocity fluctuations, as well as the entropy jump in the simulation,
can be reproduced by mixing-length theory for a ratio of mixing length
to pressure scale height α ~ 1.5. However, local mixing-length theory
neither reproduces the profiles of these variables with depth nor allows
penetration of convective motions into the overlying stable photosphere.
