X-Ray Morphology Due to Charge-exchange Emissions Used to Study the Global Structure around Mars

Soft X-ray emissions induced by solar wind ions that collide with neutral material in the solar system have been detected around planets, and were proposed as a remote probe for solar wind interaction with the Martian exosphere. A multi-fluid three-dimensional magnetohydrodynamic model is adopted to derive the global distributions of solar wind particles. Spherically symmetric exospheric H, H₂, He, O, and CO₂ density profiles and a sophisticated hybrid model that includes charge-exchange and proton-neutral excitation processes are used to study the low triplet line ratio G = i + f r (0.77 ± 0.58) of O vii and the total X-ray luminosity around Mars. We further calculate the emission factor α-value with different neutrals over wide ion-abundance and velocity ranges. Our results are in good agreement with those of previous reports. The evolution of the charge stage of solar wind ions shows that sequential recombination due to charge-exchange can be negligible in the interaction region. This only appears below an altitude of 400 km. The anonymous low disk G-ratio can be easily explained by the collisional quenching effect at neutral densities higher than 10¹¹ cm⁻³. However, the quenching contribution is small in Mars’ exosphere and only appears below 400 km. Charge-exchange with H₂ and N₂ is still the most likely reason for this low G-ratio. X-ray emissivity maps in collisions with different neutrals differ from each other. A clear bow shock arising from the collision with all the neutrals is in accordance with previous reports. The resulting total X-ray luminosity of 6.55 MW shows better agreement with the XMM-Newton observation of 12.8 ± 1.4 MW than that of previous predictions.