Effects of charge exchange and electron impact ionization on the formation of the magnetic pileup boundary at Mars

Abstract

A relatively sharp increase in the solar wind magnetic field, Magnetic Pileup Boundary (MPB), has been observed in the inner Martian magnetosheath. We examined effects of exospheric processes, such as charge exchange reactions (CE) and electron impact ionizations (EII), on the formation of the MPB by using a newly developed two‐dimensional global MHD model coupled to Monte Carlo simulations. The CE and EII processes change the energy distribution of the solar wind plasma, causing a depletion of the thermal pressure, and thus an enhancement of magnetic pressure so as to maintain the pressure balance in the radial direction. Beside decreasing the thermal pressure, these processes decelerate the solar wind by adding mass of slow ions (protons and oxygen ions). These processes were treated self‐consistently in the new MHD model. The MHD model implicitly includes the time evolution of ion and electron distributions being deformed from Maxwellian shapes under the influence of CE and EII reactions, by making use of their reaction rate coefficients that were derived as functions of “effective temperatures” from Monte Carlo simulations. The MHD simulation results suggest that an enhancement of magnetic field magnitude occurs in the inner magnetosheath mainly due to the CE effect at the solar minimum and to the EII effect at the solar maximum. EII also decelerates the solar wind significantly through the mass‐loading effect, which lengthens the interaction time between the solar wind plasma and the exospheric atoms, resulting in an additional enhancement of magnetic field magnitude both at the solar minimum and maximum. It was also found that these exospheric processes significantly influence the ion density profiles above the upper ionosphere. The large and nearly constant scale height of total ion density as observed by past measurements can be caused by the EII process. Moreover, EII produces a large number of O+ ions above the ionopause especially at the solar maximum, which is potentially important in terms of atmospheric escape.