Lyman-α emission in the Martian proton aurora: Line profile and role of horizontal induced magnetic field

Enhancements of the dayside Lyman-alpha emission by as much as similar to 50% have been observed between 120 and 130 km in the lower Martian thermosphere from the Mars Express and MAVEN satellites, usually following solar events such as coronal mass ejections and corotating interaction regions. They have been assumed to be optical signatures of proton aurora related to an increase in the solar wind proton flux hitting Mars' bow shock. We present model simulations of the Lyman-alpha line profiles at different altitudes. These are partly guided by in situ measurements of the energy spectrum of protons in the magnetosheath region by the SWIA instrument on board the MAVEN spacecraft. We show that the auroral Lyman-alpha line profile is significantly broader than the hydrogen core of the planetary thermal H atom. Consequently, most of the auroral emission is produced outside the optically thick hydrogen core and creates the observed intensity enhancement. Simulations with incident energetic hydrogen atoms (H ENAs) produce a somewhat broader line profile. Monte Carlo calculations are made separately for incident solar wind protons and H ENAs produced by charge exchange in the hydrogen corona. Absorption by CO2 along the line of sight significantly affects the intensity distribution in the lower thermosphere. The calculated altitude of the peak emission for both types of incident particles is consistent with the observed characteristics of the proton aurora. We show that the presence of a horizontal induced magnetic field somewhat increases the line width and decreases the altitude of the emission peak as a consequence of the magnetic barrier effect on proton precipitation. The brightness of the Lyman-alpha emission also drops as a result of increased magnetic shielding of the protons.