Cornelis ("Kees") Zwaan died in his house at Doorn, The Netherlands,
on June 16, 1999 from cancer. He underwent major surgery in March
1998 but recovered so fast that we and his other friends expected to
have him with us much longer. It is hard to realize that he is gone

Rutten, R.J.

Schrijver, C.J.

Cornelis ("Kees") Zwaan died in his house at Doorn, The Netherlands,

on June 16, 1999 from cancer. He underwent major surgery in March

1998 but recovered so fast that we and his other friends expected to

have him with us much longer. It is hard to realize that he is gone

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C. Zwaan 1928-1999

Utrecht University Repository

C. Zwaan 1928 { 1999Solar Physics volume 188, page 1{2, Klwer, Dordrecht, 1999Cornelis (\Kees") Zwaan died in his house at Doorn, The Netherlands,on June 16, 1999 from cancer. He underwent major surgery in March1998 but recovered so fast that we and his other friends expected tohave him with us much longer. It is hard to realize that he is gone.Zwaan studied physics and astronomy at Utrecht University during1946{1954 and then became a physics teacher during ve years, inter-rupted by military service. In fact, Zwaan was a life-long educator, asa boy teaching bird and plant lore to his younger brother Joost, as aprofessor teaching stellar atmospheres and radiative transfer to manygenerations of Utrecht students, and in particular, as a PhD adviserteaching a long procession of graduate students1 how to tackle astro-physics. Many of the latter remained in the eld | no other Utrechtastrophysicist has fathered such a \school". In addition, he inspired agreat many colleagues in their research, both at Utrecht and elsewhere.In 1965 Zwaan defended his own thesis, \Sunspot Models | AStudy of Sunspot Spectra", supervised by M.G.J. Minnaert. The con-tent is characteristic. On one hand, painstaking analysis of how scat-tered light upsets sunspot spectroscopy; on the other, scenarios of howsunspots assemble and even a sunspot-related flare mechanism | atypical Zwaan mix of both meticulous detail and bold thinking. Thecombination made his research both daring in regularly attacking newtopics, and solid in doing so with sucient depth | always with eager-ness, dedication and admirable discipline.The Zwaans (Kees married Prisca van Diggelen in 1954, an evidentlyharmonious partnership) spent 1966{1967 at Sacramento Peak, a richexperience that made them lovers of the US South-West (turning Keesinto a bolo-tie wearer to the dismay of his Dutch relatives). Aboveall, it gave Kees opportunity to study active region morphology onthe large lm collection at the Big Dome. He became an expert \solarnaturalist", recognizing structural and evolutionary patterns in solarmagnetism much as he knew his birds and plants, displaying uncanny1 Rob Rutten (1976), Henk Spruit (1977), Albert Greve (1978), Frans Middelkoop(1982), Aad van Ballegooijen (1982), Barto Oranje (1985), Hans Brants (1985), KarelSchrijver (1986), Rene Rutten (1987), Karen Harvey (1993), Louis Strous (1994) andKen Tapping (1995); as co-adviser Han Uitenbroek (1990), Jo Bruls (1992), Jos vanGeen (1993) and Nick Hoekzema (1997), at the University of Amsterdam Jan vanParadijs (1975) and Ankie Piters (1995).2intuition to the less experienced. Also, the American frankness suitedKees and Prisca well | many of their colleagues-turned-friends arefrom overseas, and many have enjoyed splendid hospitality at Doorn.Zwaan became university lecturer (associate professor) upon hisreturn to Utrecht in 1968, full professor in 1980. The early seventies sawlarge involvement in JOSO, then at the heydays of site selection. Zwaaninspired Rob Hammerschlag to start on his open solar telescope, a longdevelopment that nally came to rst light in 1997. Together, theysite-tested sandy beaches o the Algarve and Zwaan even inspectedthe tiny Ilhas Selvagens, championing such sea-level sites, but eventu-ally the JOSO telescopes went to the Canary Island volcano’s. In themeantime, Zwaan, Nigel Weiss and Aad Fokker had combined JOSOand CESRA into the Solar Physics Section of the European PhysicalSociety with Zwaan as founding chairman.Zwaan’s research with his students concentrated initially on NLTEline formation and sunspot spectroscopy but diversied soon, expandingto both solar and stellar magnetic activity as a two-way street utilizingtheir complementarity. His interests ranged from small-scale structuresto stellar dynamos and from intrinsically weak magnetic elds in thesolar photosphere to the merging of magnetically-braked binaries. Hewas directly involved in the development of the fluxtube paradigm,stimulated discussions of a boundary-layer dynamo, initiated cool-starpolarimetry, pioneered in diagnosing sunspot nests, and led a sequenceof coworkers in studies of stellar chromospheric activity including par-ticipation in the Mount Wilson H&K monitoring program.Zwaan contributed to many papers, but rarely as rst author andoften satised with just an acknowledgment. Browsing through the on-line ADS compilations of who cited which Zwaan paper through theyears quickly shows that his citation record ranks very high and that itcovers an impressive variety of topics. Stellar magnetism is the threadlinking them together, but they range widely across solar and stellarastrophysics.Zwaan completed his last major project only recently: co-authoringa textbook \Solar and Stellar Magnetic Activity" with one of us (CJS).It will appear this autumn and is intended as an introductory overviewfor graduate students and non-solar astrophysicists. Most of the pageswritten by Zwaan convey his awe for the inspiring questions that stellarmagnetism furnishes to all of us as it did to him throughout his career.We are fortunate that his voice will live on through these pages.R.J. Rutten, UtrechtC.J. Schrijver, Palo Altosolphys.tex; 9/02/2000; 12:43; no v.; p.2

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C. Zwaan 1928-1999

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