Utrecht solar physics

Utrecht solar physics = Dutch solar physics is dead.  It abruptly transited from high international standing to nothing.  At present I remain the only researcher active in solar physics in The Netherlands (excepting international scientists at ESA's ESTEC facility and venerable Kees de Jager on Texel).

The motto of Utrecht University is Sol iustitiae illustra nos ("The Sun of justice enlightens us").  It should be:  Sol scientiae fabula est ("The Sun of science is gone").  As is Utrecht solar physics.  In 2007 Utrecht University stopped supporting the Dutch Open Telescope because the other Dutch universities did not share interest in solar physics.  In 2011 Utrecht University killed all its astronomy because the other major Dutch universities (at Leiden, Amsterdam, Groningen and Nijmegen) do astronomy too.  In profiling itself as differing from them and targeting "useful" science only, Utrecht University could/should have revived its former specialistic eminence in solar physics and space-weather research - the branch of astrophysics with direct economical relevance - instead of scrapping also this world-famous expertise.  I suspect that the board of Utrecht University wasn't even aware that they killed this field together with night-time astronomy.

The legacy of Utrecht solar physics lives on in the form of the many Utrecht alumni who are active in solar physics research elsewhere.  They collectively embody the great name that Utrecht University had in this field but discarded.  At the time of closure (2011, in PhD order): Henk Spruit (Germany), Aad van Ballegooijen (USA), Piet Martens (USA), Karel Schrijver (USA), Paul Hick (USA), Han Uitenbroek (USA), Jo Bruls (Germany), Martin Volwerk (Austria), Kostas Tziotziou (Greece), Luc Rouppe van der Voort (Norway), Michiel van Noort (Germany), Alfred de Wijn (USA), Jorrit Leenaarts (Norway), Nikola Vitas (Spain), Catherine Fischer (ESA), Gregal Vissers (Norway), Tijmen van Wettum (Germany).  Plus (at the time of closure) active-pensioner Utrecht alumni Kees de Jager (Texel), Jacques Beckers (USA), and myself (Lingezicht).

Utrecht solar physics has a rich history, with Minnaert, de Jager, and Zwaan as successive protagonists.  It started a century ago with solar spectroscopy (including eclipse expeditions) by J.H. Julius at the Utrecht physics laboratory.  His collaborator M.G.J. Minnaert, a most eminent scientist and outstanding person, had fled from Belgium (with a PhD in biology) to escape forced-labor enprisonment as convicted Flamingant.  Minnaert revived Sterrewacht Sonnenborgh in the 1930s (choosing not to become director of Yerkes Observatory; Chandrasekhar got that job instead) and turned it into a well-known astrophysics institute specialising in solar spectrum analysis.  He moved Julius' solar telescope and spectrograph to the observatory and developed the concepts of equivalent width and curve of growth to measure and interpret the strength of Fraunhofer lines, laying the basis for quantitative astrophysical spectrometry.  In those years he also completed the three volumes of his famous outdoor physics trilogy.  The Dutch originals ("De natuurkunde van 't vrije veld") are now available, with biographies and other material, at the DNBL digitale bibliotheek.

The famous Utrecht Atlas of the solar spectrum was completed just before the second world war.  The spectrum plates were taken by G.F.W. Mulders at Mount Wilson.  (Mulders later became a director of the US National Science Foundation while another Minnaert pupil, J.H. Bannier, became the first director of the Dutch science foundation (ZWO) and an important architect for ESRO (later ESA), ESO, and CERN.)  The plates were scanned by Minnaert and J. Houtgast with an ingenious version of Moll's microdensitometer that applied analog optical conversion of emulsion opacities into solar intensities, as illustrated in the prefaces to the atlas (two, in English but also in Esperanto, one of Minnaert's many languages).

In 1942 (the year I was born) Houtgast defended his landmark PhD thesis establishing the importance of frequency redistribution in solar line formation.  Minnaert was his supervisor but was then taken hostage by the Germans.  A year later Houtgast permitted students C. de Jager and H. Hubenet to escape deportation to Germany by hiding at the observatory, of which they had the run at night.

Towards the end of the war, spectroscopic expertise gained by studying the telluric oxygen lines in the Atlas enabled H.C. van de Hulst to answer J.H. Oort's question whether useful spectral lines exist at radio wavelengths.  He asked Léon Rosenfeld (then professor at Utrecht) for advice, was pointed to hydrogen ground-state hyperfine structure, and predicted that the corrresponding hydrogen 21-cm line should be suited for then-infant galactic radio astronomy.  By that time solar/stellar spectroscopy was a quantitive science, but radiative transfer was too complex for further analytical development and awaited the advent of computers in the 1960s.  Van de Hulst moved to Leiden and turned to scattering in planetary atmospheres for which progress could still be made analytically.

After the war Minnaert and Houtgast undertook the gigantic task of measuring all Fraunhofer lines in the Atlas, with everybody at the Sterrewacht participating as so-called "computer".  Charlotte E. Moore (US Naval Research Laboratory) contributed the identifications from her Revised Multiplet Table (ADS 1945CoPri..20....1M).  Here is the Moore-Minnaert-Houtgast table of the solar spectrum (ADS: 1966sst..book.....M).

C. de Jager took over from Minnaert after his thesis on solar flare spectrometry using the Sonnenborgh spectrograph.  He brilliantly led a fast expansion adding groups in solar radio astronomy (A.D. Fokker), solar plasma astrophysics (M. Kuperus), and X-ray solar-flare research (at SRON).  After his mandatory retirement in 1986 he concentrated on giant stars; after returning to Texel on solar climate forcing.  Houtgast kept to flash-spectrum photography at eclipses taking me to Greece, Brazil, and Mexico.  C. Zwaan continued Minnaert's optical solar physics tradition adding emphasis on solar magnetism, sun-inspired cool-star activity research, and inspired R.H. Hammerschlag to build the Dutch Open Telescope.  Chromospheric physics with the latter was my main emphasis during the final decade of Utrecht solar physics.  I also followed Zwaan's example in teaching spectral line formation to hundreds of students.  At my mandatory retirement in 2007 C.U. Keller (spectropolarimetry) and A. Vögler (numerical magnetohydrodynamics) were the solar physicists on the SIU staff.  At the closure they and their graduate students and postdocs, engineers F.C.M Bettonvil, A. Jägers and G. Sliepen, and active-pensioners R.H. Hammerschlag and me constituted the largest group in the SIU.  Since then Dutch solar physics consists mainly of me working at home.

List of Utrecht University PhD's with solar physics as thesis subject:
    J.B. Hubrecht (1915):  The solar rotation in June 1911 from spectrographic observations
    A.S.P.J.M. Wanders (1933):  Onderzoekingen over de straling der zonnevlekken
    G.F.W. Mulders (1934):  Aequivalente breedten van Fraunhoferlijnen in het zonnespectrum
    A.P.H. van der Meer (1936):  Onderzoek naar variaties in de aequivalente breedten van Fraunhoferlijnen in het zonnespectrum
    J. Houtgast (1942):  The variations in the profiles of strong Fraunhofer lines along a radius of the solar disc
    W.J. Claas (1951):  The composition of the solar atmosphere
    C. de Jager (1952):  The hydrogen spectrum of the sun
    H. Hubenet (1960):  The influence of the photospheric model on the determination of the solar composition
    A. Schadee (1964):  The formation of molecular lines in the solar spectrum
    J.M. Beckers (1964):  A study of the fine structures in the solar chromosphere
    M. Kuperus (1965):  The transfer of mechanical energy in the sun and the heating of the corona
    J.R.W. Heintze (1965):  The extreme limb of the sun
    C. Zwaan (1965):  Sunspot models, a study of sunspot spectra
    T. de Groot (1966):  Weak solar radio bursts
    L.D. de Feiter (1966):  Analysis of the Balmer spectrum of solar flares
    J. Roosen (1968):  Some features of the solar microwave emission
    J. van Nieuwkoop (1971):  A multi-channel solar radio spectrograph
    A.C. Brinkman (1972):  Instrumentation for the detection of solar soft X-rays and the analysis of a solar event
    H.F. van Beek (1973):  Development and performance of a solar hard X-ray spectrometer
    J. Rosenberg (1973):  Instabilities in the solar corona
    J.M.E. Kuijpers (1975):  Collective wave-particle interactions in solar type IV radio sources
    P. Hoyng (1975):  Studies on hard X-ray emission from solar flares and on cyclotron radiation from a cold magnetoplasma
    R.J. Rutten (1976):  Solar eclipse observations and Ba II line formation
    H.C. Spruit (1977):  Magnetic flux tubes and transport of heat in the convection zone of the sun
    W.J. Weber (1978):  The dynamics of coronal magnetic structures
    A. Greve (1978):  High resolution UV observations and the formation of the solar Mg II resonance lines
    W. van Tend (1979):  Magnetohydrodynamic and thermal processes in solar flare energy build-up and release
    A. Kattenberg (1981):  Solar radio bursts and their relation to coronal magnetic structures
    C. Slottje (1982):  Atlas of fine structures of dynamic spectra
    F. Middelkoop (1982):  Ca II H and K emission from late-type stars
    N.P.M. Kuin (1982):  Stellar coronae and their mass loss
    A.A. van Ballegooijen (1982):  Sunspots and the physics of magnetic flux tubes in the sun
    P.C.H. Martens (1983):  Non linearity and instability in stellar coronae
    A. Duyveman (1983):  X-Ray imaging and interpretation of impulsive solar flare phenomena
    J.S. Kaastra (1985):  Solar flares, an electrodynamic model
    B.J. Oranje (1985):  Solar-type stellar chromospheres, an observational study of the outer atmospheres of cool stars
    J.J. Brants (1985):  Observational study of the birth of a solar active region
    C.J. Schrijver (1986):  Stellar magnetic activity, complementing conclusions based on solar and stellar observations
    G.H.J. van den Oord (1987):  Stellar flares
    P.P.L. Hick (1988):  Interpretations of energetic phenomena in the solar corona
    H. Uitenbroek (1990):  Partial redistribution modeling of the Ca II K line
    J.H.M.J. Bruls (1992):  Formation of diagnostic lines in the solar spectrum
    J.H.G.M. van Geffen (1993):  Magnetic energy balance and period stability of the solar dynamo
    K.L. Harvey-Angle (1993):  Magnetic bipoles on the Sun
    M. Volwerk (1993):  Strong double layers in astrophysical plasmas
    L.H. Strous (1994):  Dynamics in solar active regions
    K.F. Tapping (1995):  Discrete microwave sources in solar active regions
    A.J.H. Ossendrijver (1996):  Fluctuations and energy balance in solar and stellar dynamos
    K. Tziotziou (1997):  Dynamics of stellar coronae
    N.M. Hoekzema (1997):  Statistical studies of dynamical structures in the solar atmosphere
    N.A.J. Schutgens (1998):  Oscillating prominences
    H.J. Hagenaar (1999):  Flows and magnetic patterns on the solar surface
    J.M. Krijger (2002):  Structure and dynamics of the solar chromosphere
    A.G. de Wijn (2006):  Dynamics of fine structure in the solar chromosphere
    J. Leenaarts (2007):  Numerical simulations of the solar atmosphere
    F. Snik (2009):  Astronomical polarimetry: new concepts; new instruments; new measurements & observations
    N. Vitas (2011):  Observational signatures of the simulated solar photosphere
    C.E. Fischer (2011):  Transient events in the solar photosphere at high spatial and temporal resolution

Rob Rutten 2017-12-12