Bob Marshall: Investigating Space Plasmas

New AES Assistant Professor Dr. Robert Marshall’s interests and expertise are unquestionably diverse. Spanning small satellite development to lightning-ionosphere interactions, Marshall’s research aims to shed insight on the nature of space plasmas and their affects on spacecraft and wave propagation.

Marshall’s involvement in space physics emerged from a background in electrical engineering. He explains:

“During my undergraduate at the University of Southern California, I focused on ‘electrical engineering stuff,’ including amplifier and circuit design. But my main area of interest was electromagnetics and wave propagation. In graduate school, I was pulled into Stanford University’s Very Low Frequency (VLF) Research Group, which has long had a focus on space physics. VLF had the electromagnetics slant that I was looking for, along with a plasma physics aspect that I found fascinating.”

Since joining the VLF group, Dr. Marshall has acquired extensive experience with small satellite and sensor development. Beginning in 2006, the VLF Group was charged with constructing a VLF wave transmitter and receiver for inclusion on the Air Force Research Lab’s DSX mission, which aims to show the viability of operating spacecraft in the harsh environment of Earth’s radiation belts. For Marshall, the design and management of DSX’s CubeSat companion, the VLF wave and Particle Precipitation Mapper (VPM), was his first introduction to spacecraft design:

“The VLF group was building spacecraft receivers for DSX, and there was a lack of personnel in the group to tackle VPM - so I jumped into it. It was my first exposure to spacecraft design, and it gave me the opportunity to design instruments for making wave and particle measurements in the radiation belts.”

Since the delivery of the VPM CubeSat in January 2015, Marshall’s attention has shifted to the development of a small satellite that will host an active electron accelerator. He notes:

“Our idea is to put an accelerator on board to send out a beam of high-energy electrons. By seeing how these electrons bounce around in the space environment, we will get a better idea of how the natural radiation belt electrons are affected by the plasma surrounding the Earth.”

There is no shortage of electrons and electromagnetic waves already freely bouncing around in Marshall’s area of interest. However, because the sources of these waves and electrons are not well known, the effect of the plasma on their properties cannot be easily deduced. By producing electrons via an electron accelerator, Marshall and his team can specify the electrons’ initial conditions and thus quantify the plasma-induced changes to their speed, energy and pitch angle.

Coupling Marshall’s experience with small satellites is an expertise in lightning-upper atmosphere interactions, specifically electromagnetic phenomena such as sprites, elves, and whistler-mode waves. He explains:

“My research has been the interaction between lightning and the upper atmosphere and lower ionosphere. I am currently investigating how electromagnetic waves from lightning (known as whistler-mode waves) can be used to probe the radiation belts. Electromagnetic waves are quite peculiar. When they propagate through a vacuum, they are radio waves propagating at the speed of light. However, when they move into the ionosphere, they turn into plasma (whistler) mode waves, and propagate at a fraction of the speed of light. The characteristics of these whistler-mode waves can be used to deduce information about the plasma environment of near-Earth space.”

 

Though Marshall’s research may appear disparate, it is unified by a common theme:

“I am fundamentally trying to determine the effect of waves propagating in space plasmas. We often treat space as a ‘vacuum.’ However, this is not entirely correct. All the way out to geo-synchronous orbit, the near-Earth space environment is filled with plasma. If you put a satellite out there and are transmitting to the Earth at lower frequencies, your waves will be affected by this plasma.”

As Marshall prepares to begin his faculty career at CU this fall, he sees his research naturally complimenting existing investigations:

“While I will have my own research interests, I anticipate many faculty collaborations. Already, I have been working with [CCAR researchers] Jeff Thayer and Jeff Forbes on their ‘global electrical circuit’ research studying how electrons travel cyclically around the atmosphere and ionosphere via thunderstorms and fair weather current.  In the future, I anticipate collaborating with [AES Professor] Scott Palo on his small satellite and meteor research, [AES Professor] Xinlin Lee on radiation belt work, and [Electrical Engineering Professor] Al Gasiewski on atmospheric science inquiries.”

In light of these various collaborations, Marshall reflects on the appeal of the CU-Boulder research community:

“CU-Boulder is one of the preeminent places in the world for doing small satellite and space physics research. Between and the expertise brought by faculty in AES, the Laboratory for Atmospheric and Space Physics (LASP), and many other departments and institutes on campus, the depth and breadth of space physics knowledge here is truly unparalleled. I’m also excited to be a part of the university as it takes on the Grand Challenge, which is helping to make Boulder the place that students think of when they think of space research.” 

-Written By: Ari Sandberg, Intern