A Monte Carlo Model of Crustal Field Influences on Solar Energetic Particle Precipitation into the Martian Atmosphere

Solar energetic particles (SEPs) can precipitate directly into the atmospheres of weakly magnetized planets, causing increased ionization, heating, and altered neutral chemistry. However, strong localized crustal magnetism at Mars can deflect energetic charged particles and reduce precipitation. In order to quantify these effects, we have developed a model of proton transport and energy deposition in spatially varying magnetic fields, called Atmospheric Scattering of Protons and Energetic Neutrals. We benchmark the model's particle tracing algorithm, collisional physics, and heating rates, comparing against previously published work in the latter two cases. We find that energetic nonrelativistic protons precipitating in proximity to a crustal field anomaly will primarily deposit energy at either their stopping altitude or magnetic reflection altitude. We compared atmospheric ionization in the presence and absence of crustal magnetic fields at 50 degrees S and 182 degrees E during the peak flux of the 29 October 2003 Halloween storm SEP event. The presence of crustal magnetic fields reduced total ionization by similar to 30% but caused ionization to occur over a wider geographic area.