Effects of Solar Wind Density and Velocity Variations on the Martian Ionosphere and Plasma transport-A MHD Model Study

Solar wind dynamic pressure, consisting solar wind density nsw and velocity Vsw, is an important external driver that controls Martian plasma environment. In this study, a 3D magnetohydrodynamic model is applied to investigate the separate influences of solar wind density and velocity on the Martian ionosphere. The spatial distributions of ions in the dayside and near nightside ionosphere under different nsw and Vsw are analyzed, as well as the ion transport process. We find that for the same dynamic pressure condition, the ionosphere extends to higher altitudes under higher solar wind density, indicating that a solar wind velocity enhancement event is more efficient at compressing the Martian ionosphere. A higher Vsw will result in a stronger induced magnetic field, shielding the Martian ionosphere, preventing the penetration of solar wind particles. For the same dynamic pressure, increasing nsw (decreasing Vsw) leads to a higher horizontal ion velocity, facilitating day-to-night plasma transport. As a result, the ionosphere extends farther into the nightside. Also, the ion outflow flux is larger for high nsw, which may lead to a higher escape rate. Moreover, the strong crustal fields in the southern hemisphere also cause significant effect to the ionosphere, hindering horizontal ion transport. An additional outflow channel is also provided by the crustal field on the southern dayside, causing different responses of flow pattern between local and global scale while the solar wind condition is varied. Solar wind dynamic pressure is one of the main factors that influence Martian space environment. Variation in solar wind velocity and density can cause different effects to the magnetic field and plasma environment in Martian space. By using time dependent 3D multifluid MHD model, we studied the influence of individual solar wind velocity and density on the Martian ionosphere and plasma flow. We found that a higher solar wind density can cause an expansion of ionosphere and a higher outward flow of ions, while a higher solar wind velocity can decrease the horizontal ion velocity and day-to-night transport by enhancing the strength of induced magnetic field, thus the ionosphere extends farther into the nightside under low solar wind velocity condition. The remnant fields of Mars also cause an apparent north-south asymmetry in the ionosphere, since strong crustal fields can deflect plasma flow, hindering horizontal plasma transport while providing an additional vertical outflow channel. For constant solar wind pressure, the Martian ionosphere compresses as the solar wind velocity increasesFor constant dynamic pressure, higher solar wind density leads to higher horizontal ion velocity, facilitating day-to-night transportStrong remnant fields in the southern hemisphere uplift the Martian ionosphere and hinder horizontal ion transport