Gordon Dean Holman

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.

Radio emission from shock waves driven by solar eruptions

Eruptions of rapidly moving material from the Sun, coronal mass ejections, drive shock waves in the solar corona and beyond. Radio emission has long been observed from these shock waves. This solar radio emission was given the designation "Type II radio burst" in the early 1950s. Holman and Pesses hypothesized that Type II emission is stimulated by electrons energized by shock drift acceleration. They found the hypothesis to be sound and identified implications for the geometry of the shock wave and the corresponding sources of radio emission. Holman has collaborated on observational studies examining the initiation of coronal mass ejections and the properties of Type II emission.

Coronal magnetic field in active regions on the Sun

The Sun's magnetic field is enhanced in active regions, where sunspots are located. Major solar flares and eruptions also originate in active regions. (However, in 2015 Holman and collaborator Adi Foord found that a significant X-ray flare occurred outside any active region, in association with the eruption of a long, quiescent filament.) The enhanced magnetic field and its evolution is understood to be the source of energy for this activity. Therefore, it is important to know the strength and structure of active region magnetic fields.

The wavelength of microwave emission from active regions provides a measure of the magnetic field strength in hot, magnetically confined structures at coronal temperatures. In the late 1980s and early 1990s, Holman and collaborators carried out the Coronal Magnetic Structures Observing Campaign (CoMStOC). This campaign obtained observations of multiple active regions with the Very Large Array (VLA) radio telescope in New Mexico combined with X-ray observations from the Flat Crystal Spectrometer on the Solar Maximum Mission (SMM) satellite as well as other relevant observations. Magnetic field strengths up to 600 Gauss were deduced, while the presence of cooler plasma and higher Alfvén speeds than expected in the corona were also inferred.

Comet ion tail disconnection events

A particularly impressive feature observed in comets is the disconnection event, when the ion tail (also called gas or plasma tail) of the comet disconnects and moves away from the head. There are several possible explanations for this phenomenon, one of which is magnetic reconnection at the front of the comet where the tail magnetic field wraps around the nucleus.

In 1985 Holman contributed to the NASA Jet Propulsion Laboratory publication "The comet Giacobini-Zinner handbook. An observer's guide to the first comet to be explored by a spacecraft". The intercepting spacecraft was the International Cometary Explorer (ICE). When Halley's comet made its appearance in 1986, ICE was nearby obtaining measurements of the thermal and magnetic properties of the solar wind through which the comet was traveling. Brosius, Holman and collaborators examined two disconnection events that occurred during this period of time along with the properties of the solar wind measured by ICE. They concluded that the first disconnection event was most likely front-side magnetic reconnection caused by a polarity reversal in the solar wind magnetic field impacting the comet. The second event might have also been caused by a polarity reversal, but a high density and velocity compression region was present as well that might have caused or contributed to the event.

Energetic particles and magnetic reconnection in solar flares

Holman was Co-Investigator and member of the science team for the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The RHESSI satellite was an observatory designed to explore the basic physics of particle acceleration and explosive energy release in solar flares by obtaining high-resolution spectroscopic imaging of flares in X rays and gamma rays. It was launched in early 2002 and was decommissioned in 2018. Much of Holman's research focuses on analysis of the RHESSI X-ray data to deduce the origin and evolution of energetic electrons produced in solar flares.

Holman has collaborated on major studies of the partition of energy in solar eruptive events (large eruptions consisting of both a flare and a coronal mass ejection). Observational studies by him and his collaborators have helped establish that the primary site of energy release is magnetic reconnection in the corona, seen as a cusp at the top of flare magnetic loops. He has examined the impact of a return current that is expected to be co-spatial with the streaming energetic electrons responsible for the highest energy X-ray emission from flares and, with Meriem Alaoui Abdallaoui, continues to examine the impact of the return current on flare evolution.

Multiple reviews of this and related research have been authored or coauthored by Holman. In 2016 he authored a review addressing future observations from space that would lead to substantial progress toward our understanding of solar activity.