CAREER: A Laboratory Test of Radiation Belt Electron Acceleration and Diffusion by Whistler Chorus

Beyond Earth's atmosphere is a universe where plasma is far more abundant than solid, liquid, and gas. At the dawn of the space age, Explorer 1 unexpectedly discovered ions and electrons in space plasma traveling at nearly the speed of light. These high-velocity particles are found in two donut-shaped regions around Earth called the Van Allen Radiation Belts.

Understanding variations of the radiation belts have continued urgency as actionable space weather predictions are needed to protect critical infrastructure in space and on the ground. Measurements in the plasma of the outer belt commonly detect intense electromagnetic waves called whistler-mode waves, along with dramatic changes in the number of high-velocity electrons.

Electrons are sometimes accelerated rapidly, in less than a second, and sometimes over hours. Models suggest that whistler-mode waves can produce these varied outcomes. The amplitude of whistler-mode waves and the angle between wave crests and Earth's magnetic field are predicted to determine the nature of interactions with electrons.

The scientific objectives of this grant include a series of laboratory experiments in conditions relevant to the outer belt to study the interaction between whistler-mode waves and electrons. Work will consist of developing a new antenna array to generate whistler-mode waves while controlling the angle between wave crests and the magnetic field. Educational objectives focus on increasing participation in space plasma physics research with an early-undergraduate pathway to research that will allow students to be recruited from more diverse groups.

Research students will be supported by an introductory seminar, and seminar materials will be distributed freely to help cultivate early undergraduate research across the space plasma physics community.

With frequencies below the electron cyclotron frequency and resonant velocities often comparable to characteristic electron velocities, whistler-mode waves have potent wave-particle interactions with electrons that are central to descriptions of radiation belt dynamics. Theory and simulation indicate the amplitude of whistler-mode waves and the wave-normal angle are important variables in determining the character of interactions with electrons.

A limitation of earlier lab work was the absence of high-resolution and high-precision measurements of electron velocity distribution functions (evdf's) needed to diagnose whistler-mode wave-electron interactions. This work will combine laboratory experiments with recent diagnostic advances to make definitive measurements of whistler-mode wave-electron interactions relevant to Earth's radiation belts.

Experiments will use Thomson scattering evdf measurements and a whistler-mode wave antenna array to define the launched wave vector. Laboratory tests will answer three questions: how conditions determine whether whistler-mode waves produce rapid electron acceleration or slower diffusion; how the onset of nonlinear wave-particle interactions varies with wave-normal angle; and how quasilinear interactions responsible for diffusion vary with wave-normal angle.

Answers to these questions are required to effectively model critical radiation belt processes, including the lifetime of trapped particles, the diffusion coefficients prevalent in numerical models, and the rapid variations leading to pulsating auroras and associated microbursts of relativistic electrons.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.