Wave-Particle Interactions and Their Effect on Electron Transport on the Crustal Fields of Mars

Abstract

The study of the transport of superthermal electrons in planetary space environments is important because they are able to efficiently heat and ionize the upper atmosphere, a contributing factor in atmospheric escape and ionospheric dynamics. The hybrid magnetosphere of Mars, with characteristics of both induced and intrinsic magnetospheres, offers a unique and complicated space environment to study space physics and electron transport. The magnetic topology of Mars is a mix of interplanetary magnetic fields, localized crustal fields connected to the planet, and reconnected crustal fields that allow access of solar wind particles to the lower atmosphere. This system is highly dynamic, both spatially and temporally, as the crustal fields rotate with the planet, in and out of interaction with the solar wind. Electron pitch angle distributions, along with energy spectra, allow us to infer the magnetic topology, which is critical for the interpretation of spacecraft measurements.

Previous studies have suggested that our understanding of electron transport on the crustal magnetic fields of Mars is incomplete and that our assumptions of what pitch angle distributions we expect on closed fields are incorrect. First, using data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, I have shown that the pitch angle distributions of high energy (100-500 eV) photoelectrons are not explainable by collisions and adiabatic invariants alone, and that the plasma dynamics on the crustal fields are more complex than originally thought. I hypothesize that whistler-mode waves are preferentially energizing trapped electrons to these high energies. Second, I used MAVEN data to study the low frequency (0.03-16 Hz) magnetic wave activity in the Mars' magnetosphere and ionosphere. I found that the magnetic wave power was highest near the boundaries of the planetary-sourced plasma, i.e. the magnetosheath and lower ionosphere, and that the solar wind regulates the injection of waves from the magnetosheath into the hybrid magnetosphere. I also showed that magnetic wave power is strongest over the closed crustal field regions. However, whistler-mode waves typically have higher frequencies than those measurable with MAVEN. Therefore, third, I used quasi-linear theory to show that the space environment of Mars is conducive for whistler-mode waves to interact with photoelectrons, and that the timescales of interaction are faster than other relevant processes (i.e., collisions). Fourth, I built a new model which solves the bounce-averaged quasi-linear diffusion equation for the steady-state velocity distribution of superthermal electrons on a Mars' crustal field in order to quantify the effect of whistler-mode waves. The initial results agree quite well with the statistical pitch angle distributions observed by MAVEN, reconciling both previous data-model discrepancies.

In this dissertation, I have shown that the observed pitch angle distributions of photoelectrons on closed crustal fields at Mars indicate ubiquitous wave-particle interactions. I have also demonstrated that whistler-mode waves can be responsible for the change in the photoelectron velocity space distribution away from what collisions alone would produce. The crustal fields of Mars, smaller scale analogs to the Earth's magnetic field, offer a new system to study wave-particle interactions. Using data, theory, and numerical modeling, I am working toward a more complete picture of electron transport at Mars.