An investigation into the necessary conditions for whistler wave particle interactions with high energy superthermal electrons on Mars crustal fields

Abstract:

Pitch angle distributions of high energy superthermal electrons (>100 eV) observed at Mars show evidence of a ubiquitous energization process occurring on dayside closed crustal magnetic fields. Wave-particle interactions have been put forth as one possible explanation and in this study we investigate if the conditions are right for such a process to be occurring regularly. We show that the resonant electron energy is not only dependent on the whistler wave frequency and normal angle, but also on the characteristic energy, which is a function of the plasma environment. The characteristic energy is determined by the magnetic field strength and thermal electron density, both measured quantities by the Mars Atmosphere and Volatile EvolutioN mission. Characteristic energies observed on dayside closed crustal magnetic fields are typically either too low for resonance with high energy electrons or resonance occurs for both high and low energy (<100 eV) electrons. If the latter is occurring, then the whistler waves are likely to be propagating quasi-parallel to the magnetic field with frequencies between 0.1-1Ω_e. Time constants of the wave-particle interaction are estimated using the diffusion coefficients from quasi-linear theory, and show that the timescales of interaction are on the order of or less than Coulomb collision timescales. However, the most efficient interaction was with low energy electrons with parallel pitch angles. These results indicate that the conditions are not right for ubiquitous interaction with only high energy electrons, however more detailed modeling is needed to understand the full evolution of the electron pitch angle distributions.