Test Particle Model Predictions of SEP Electron Transport and Precipitation at Mars

Abstract

Extreme space weather events can episodically release solar energetic particles (SEPs) that precipitate into planetary atmospheres, leading to aurora and increased ionization. While the induced magnetosphere of Mars does not substantially obstruct SEP protons, the effect on SEP electrons is not known. We use a test particle model modified for relativistic electrons to model transport of 10–200 keV electrons from outside the bow shock and deep in the magnetotail of Mars. We find a substantial influence of curvature and gradient drifts on precipitation, leading to depletions in the bow shock and transport onto closed field lines. The model estimates ∼3% of incident flux precipitates into the Mars atmosphere for a typical SEP event under nominal solar wind conditions, exceeding the estimate from the guiding center path approximation without drift terms. Precipitation is globally patchy. The model estimates 55% of electrons precipitating into cusps along open field lines and 45% precipitating on closed field lines, suggesting drift and nonadiabatic transport mechanisms have a significant influence on precipitation. We also predict that the fraction of precipitating differential flux increases as a function of energy. We discuss how these predictions would be affected by disturbed conditions that tend to accompany the arrival of SEPs and by differing SEP electron spectra hardness and anisotropy. We finally discuss how the simulation predictions compare to observations of diffuse aurora.

Modeling of Diffuse Auroral Emission at Mars: Contribution of MeV Protons

“…The sum of the electron‐ and proton‐induced CO 2 + UVD emission profiles displays similar shapes and altitude peaks as those of the observed profiles (Schneider et al., 2015, 2018), and extension of energy up to 500 keV for electrons and 20 MeV for protons enabled us to obtain the emission profiles that are more similar to the observations. However, the calculated intensity is larger than the observed intensity by a factor of 2 during the December 2014 SEP event, a discrepancy that might be explained by SEP shadowing (Lillis et al., 2016), calculation geometry effect, and magnetic mirror effect (Jolitz et al., 2021). Therefore, the contribution of energetic protons help to reconcile the in situ observations of the SEP electrons and proton fluxes onboard MAVEN with the observed emission brightness observed by MAVEN/IUVS (Schneider et al., 2015, 2018) during the two SEP events.…”

“…We did not take into account the effects of the magnetic field. Electrons are expected to be guided to the regions of open magnetic field lines, and they are unlikely to penetrate into closed field line regions (Jolitz et al., 2021; Lillis et al., 2011). Proton penetration to low altitudes is also expected to be depleted in regions of strong crustal fields (Leblanc et al., 2002).…”

“…The geometric effect is effective for only protons with energies larger than MeV, so the emission rate due to these protons could be reduced by a factor reaching ∼2. For electrons, only a few percent of the SEP electrons can reach the atmosphere due to the magnetic mirror effect (Jolitz et al., 2021), which might be applicable to low‐energy protons.…”

Energy Input of EUV, Solar Wind, and SEPs at Mars: MAVEN Observations During Solar Minimum

“…However, this would require improvement on our approach to mitigate SEP instrument contamination. Both SEP ion and electron fluxes can be shadowed or depleted at low altitudes (Jolitz et al., 2021; Lillis et al., 2016). We would need to identify and remove these events so that the estimated total flux is not underestimated.…”

“…As mentioned before, SEP ions are unhindered by all but the strongest magnetized regions of Mars, so we will assume that 100% will precipitate into the nightside. On the other hand, a test particle model of SEP electron transport estimates that ∼4% of SEP electrons precipitate on the nightside of Mars (Jolitz et al., 2021).…”