Gordon Dean Holman is an American astrophysicist at the National Aeronautics and Space Administration's (NASA's) Goddard Space Flight Center in Greenbelt, Maryland. His research mostly focused on obtaining an understanding of high-energy radiation from astronomical objects. This radiation cannot be observed from Earth's surface, but is observed with instruments on satellites launched to orbits above Earth's atmosphere. It is primarily emitted by high-energy electrons interacting with ions. These electrons also emit radiation at radio frequencies which is observed from Earth's surface. Consequently, these observations from space and radio telescopes provide a view of hot gas and energetic particles in the Universe that could not otherwise be obtained. Holman has specialized in the interpretation of these observed emissions to determine the origin and evolution of this hot gas and energetic particles. Holman spent most of his career as a solar physicist on the Science Team of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), a space observatory designed to observe X-ray emission from solar flares. He has authored or co-authored over a hundred research papers about solar flares and active regions, clusters of galaxies and disconnection events in comet tails. He is author of the book "The Scientific Method: Why science is a crucial process for human progress, not just another academic subject or belief".
Education After graduating from Northeast High School in Fort Lauderdale, Florida, in 1967, Holman obtained a Bachelor of Science degree in physics with a minor in mathematics at Florida State University in 1971. In the summer of 1970 Holman was awarded an International Association for the Exchange of Students for Technical Experience (IAESTE) traineeship at the Physikalisch-Techniche Bundesanstalt in Braunschweig, Germany, where he interned with a group studying the stimulated emission of trapped electrons from solid surfaces. This led to a research note in the journal Physica Status Solidi. Holman obtained a Master of Science degree in physics in 1973 and Doctor of Philosophy degree in astrophysics in 1977, both from the University of North Carolina at Chapel Hill. His PhD dissertation, obtained under the supervision of Dr. Wayne A. Christiansen, was titled "Models for X-Ray and Radio Emission from Clusters of Galaxies".
Career Holman started his career as a physics graduate student teaching observational laboratories for freshman astronomy students at the Morehead Planetarium and Science Center in Chapel Hill, NC. He also assisted with lecture courses in astronomy, physics, and Physics and Society. In 1977 Holman was hired as a lecturer in the Astronomy Program at the University of Maryland, College Park. In August 1977 he was awarded a two-year Postdoctoral Fellowship at the Maryland Center for Fundamental Physics. He was hired as a Research associate in 1979 by Mukul Kundu to work with his solar radio astronomy group. He remained with this group at the University of Maryland for four years. With Kundu he coedited the Proceedings of the International Astronomical Union (IAU) Symposium No, 107, "Unstable Current Systems and Plasma Instabilities in Astrophysics". In 1983 Holman was awarded a two-year National Academy of Sciences / National Research Council Senior Research Associateship at NASA's Goddard Space Flight Center. He was hired into a federal civil service position at Goddard in 1985 in what is now the Laboratory for Solar Physics, where he remained until he retired in 2018. In 1988 Holman spent roughly six months at NASA Headquarters in Washington DC assisting with NASA's Space physics proposal review process. While at NASA Goddard, Holman supervised four PhD students (through UNC Chapel Hill and The Catholic University of America) and mentored numerous Postdoctoral Fellows and summer interns. He contributed to the NASA / UC Berkeley Lesson Series Exploring Magnetism. Holman is a member of the American Astronomical Society (AAS), American Geophysical Union (AGU), International Astronomical Union (IAU), and the Committee on Space Research (COSPAR). He was an elected member of the SPD Committee of the Solar Physics Division (SPD) of the AAS in 2005 – 2007.
Research
Hot gas and energetic electrons in giant clusters of galaxies In the early 1970s extended sources of X-ray emission were detected in large clusters of galaxies with the Uhuru satellite. Extended radio sources were also observed in some large clusters such as Coma and Perseus. In his PhD Thesis Holman explored the implications of models that explained both sources as arising from the same population of energetic electrons. Subsequent observations in the mid-1970s indicated that the X-ray emission is thermal radiation from hot, ionized thermal gas (plasma) with a temperature near 100 million degrees, not more energetic non-thermal electrons. Holman and collaborators showed how the population of non-thermal electrons responsible for the radio emission could also heat the plasma producing the X-rays, providing a connection between the two sources. It had been argued that energetic charged particles streaming through magnetized thermal plasma could only propagate at the Alfvén speed because of interaction with Alfvén waves in the plasma. This would restrict the size of the radio source. Holman and collaborators showed that the damping of short-wavelength Alfvén waves could keep the particles from being slowed to this speed, especially in the exceptionally hot plasma in the clusters of galaxies.
Maser emission from the Sun and active stars In 1978 and later rapid spikes of microwave radiation were observed during the impulsive phase of some solar flares. The high brightness of these spikes indicated that a coherent radiation process must be responsible. Holman and colleagues showed that maser emission from a distribution of high-energy electrons partially mirrored at the bottom of a magnetic loop could be responsible for these spikes. This mechanism is also likely to be responsible for bright radio flares from active stars such as the M-dwarf star AD Leo.
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