Effects of solar wind density and velocity variations on the Martian ionosphere and plasma transport

Solar wind dynamic pressure, consisting solar wind density and velocity, 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 solar wind density and velocity conditions 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 solar wind velocity 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 solar wind density (decreasing velocity) 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 solar wind density, 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.