[P-CG19] Planetary Magnetosphere, Ionosphere, and Atmosphere

[PCG19-05] Study of variation mechanisms of the Mars diffuse aurora based on a Monte Carlo method

*Taishin Okiyama¹, Kanako Seki¹ (1.SCHOOL OF SCIENCE THE UNIVERSITY OF TOKYO)

Keywords:Mars, Aurora, Solar energetic particle, Monte Carlo, Induced magnetosphere

The diffuse aurora at Mars (e.g., Schneider et al. 2015) is considered to be caused by solar energetic particles (SEPs) penetrating into the Martian atmosphere along the interplanetary magnetic field lines draped around the planet. The diffuse aurora emission consists of significant CO₂⁺ ultraviolet doublet emission and having peak below 100 km altitude. Schneider et al. (2018) showed that the time variation of the auroral emission does not always correlate with the variation of the SEP flux. The emission correlates also with SEP protons in some events. The cause of the time variations of the auroral emission is far from understood. The horizontal induced magnetic field is developed when interplanetary magnetic field is draped around the Mars, and the structure of the induced magnetic field will change the flux of the penetrating SEPs. Therefore, one of the candidate mechanisms to cause the auroral variations is that change in the magnetic field orientation around Mars affects the vertical auroral emission profile. On one hand, the density profile of Martian atmosphere can change with the latitude, longitude, and local time (Slipski et al. 2018). Thus another candidate is the change in the Martian atmospheric density. The purpose of this study is to investigate effects of the two candidates on the vertical emission profile of Martian diffuse aurora based on a Monte Carlo simulation.
We have developed a Monte Carlo model that calculates the vertical emission profile of CO₂⁺ UVD, which is a typical emission line of the diffuse aurora. Our model is based on the model by Bhardwaj & Jain. (2009), which calculates the energy degradation of electrons below 1000 eV through collisions between CO₂ and electrons. The energy range of our models is expanded up to hundreds of keV by including the cross sections for collisional reactions between electrons and neutral atmosphere used in the model by Gérard et al. (2017), which reproduces vertical emission profiles of Martian diffuse aurora. A difference of our model from the previous models is to trace the trajectory of each electron in the given magnetic field structure including its cyclotron motion to investigate the effect of the draped magnetic field. The results of our model showed that the peak altitude of the emission intensity decreases with increasing elevation angle of the magnetic field from the horizontal direction. Effects of the magnetic field strength are smaller than those of the elevation angle. Difference in the altitude profiles of the neutral atmospheric density changes the peak auroral emission altitude. The result suggests that the magnetic field orientation in the vicinity of the planet and the neutral atmospheric density altitude profile are the important factors to cause variations of the vertical emission profile of the Martian diffuse aurora.