Chang-Yun (Charlie) Fan
January 7, 1918 - January 21, 2009

Dr. Chang-Yun "Charlie" Fan, a Fellow of the American Physical Society, who contributed significantly to diverse sub-fields of physics, died quietly at home in Tucson, Arizona after 91 active years of health and happiness, marred only by a final, brief battle with cancer. He will be remembered for his charm and gentle good humor by a host of colleagues, former students and friends, many of whom celebrated his long life and scientific achievements in January 2008 with a symposium and banquet in his honor entitled "Fanfare!"

Born into a peasant family in Jiangsu Province, China in 1918, through hard work, he gained entrance in 1936 to the National Central University (NCU) in Nanjing, which, like many universities, was forced to move west to escape the invading Japanese Army. Faculty, staff and students of NCU, covering some stretches by foot, settled in Chongqing, the wartime capital, where Charlie completed his BS in Physics in 1941. He stayed on in the Physics Department as a teaching assistant for several years. Savings enabled him to travel in 1947 to The University of Chicago, an institution that attracted many Chinese physics students of his generation, which included T. D. Lee and C. N. Yang. There he was awarded a PhD in Physics in 1952 under Herbert I. Anderson for his work on beta-decay and internal conversion (Phys. Rev. 87, 252, 1952). He began to contribute to the theory of the origin of cosmic rays while still a doctoral student, and continued in ensuing years (Phys. Rev. 79, 912, 1950 and 82, 211, 1951; Nuovo Cimento Suppl. 10 (8), 457, 1958).

Fan pursued post-doctoral work from 1952 to 1957 as Research Associate with Aden Meinel at the Yerkes Observatory of The University of Chicago’s Department of Astronomy and Astrophysics in Wisconsin. He built and utilized the “aurora gun” to study the emission spectra of atmospheric gases excited by both protons and electrons. His results provided a needed understanding of auroral emission spectra. Meinel remembered him recently: “Charlie discovered the high degree of variability of the appearance of H-alpha in the aurora, a puzzle that inspired similar research at the Geophysical Institute in Fairbanks. Research toward explaining this behavior is still active 45 years later, now using balloons and satellites, and the answer still being debated.” Charlie was the first to connect this high degree of variability in the aurora to geomagnetic disturbances (Astrophys. J., 128, 420, 1958).

In 1957, Fan accepted the position of Assistant Professor of Physics at the University of Arkansas in Fayetteville, where he continued his work on aurora, atomic physics and theories regarding the origin of cosmic rays.

A year later, he returned to the University of Chicago to join the John A. Simpson group as Senior Physicist and Research Associate Professor. He remained at the Laboratory for Astrophysics and Space Research (LASR) until 1967. The first decade of the Space Age was marked by Charlie’s contributions (together with Peter Meyer and John Simpson) developing space-borne cosmic-ray telescopes that used multiple solid-state detectors in coincidence. This work contributed significantly to the discoveries of the spatial separation of protons from electrons in radiation belts, interplanetary shocks, elemental and isotopic composition of cosmic rays and its modulation by the Sun, as well as the discovery of the “anomalous cosmic rays”, which eventually led to our understanding of the interaction between the solar wind and the local interstellar medium. Together with George Gloeckler, Dieter Hovestadt and others, Charlie contributed to the development of a new generation of charged-particle analyzers using thin carbon foils and electrostatic analyzers, including the Low Energy Charged Particle (LECP) instrument on Voyager 1 and 2 (Space Sci. Rev., 21, 329, 1977). Space exploration with new instruments did not take Charlie away from his love for atomic physics, as he measured the Lamb Shift in 6Li2+ (Phys. Rev. Lett., 15, 15, 1965).

In 1967, he joined the Physics Department of the University of Arizona, where he served as Professor of Physics until his retirement in 1988. During this period, while continuing his work in space physics, he expanded his research in collaboration with other groups on campus; e.g.: measuring Lamb Shift in 16O2+ with the Beam-Foil Group (Phys. Rev. Lett., 28, 1612, 1972); detecting cosmic-ray showers in Cerenkov light with the Smithsonian Observatory (15th International Cosmic Ray Conference, Conference Papers 8, 233, 1977); and studying cosmic-ray variability and astrophysical events using carbon dating with the Tree Ring Lab (16th International Cosmic Ray Conference, Conference Papers 1, 106, 1979), to name a few.

Starting in 1972, Fan developed close collaborations with several groups in China. Projects he initiated there include the study of cosmic rays using carbon dating of tree rings at different latitudes (18th International Cosmic Ray Conference, Conference Papers 3, 82, 1983), collection of cosmic dust on NASA’s Space Shuttles, and analysis of isotopic ratio of 6Li/7Li in lunar samples.

Charlie’s scientific career lives on in the work of the many former students and younger collaborators who considered him their beloved teacher and mentor. To his last day, Charlie invariably showed a genuine interest in all people and always encouraged and challenged them to bring out their best. A few days before he died, one of his oldest friends from the Yerkes days visited him. He dismissed talk about himself with one quiet remark and they talked science as usual. Charlie Fan approached both life and death with a clear mind, dignity, humor, tolerance, curiosity, courage, and a deep love for family and friends. He showed us how to live and showed us how to die.

Davis Hartman, George Gloeckler, Neil Gehrels, Jacques L’Heureux, K. C. Hsieh

Submited to Physics Today to be published online.

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