Effects of EUV on Day‐To‐Night Transport in the Martian Ionosphere

Solar extreme‐ultraviolet (EUV) radiation, as the primary ionization source of Mars' upper atmosphere, plays a vital role in determining ion density and transport in the Martian ionosphere, although its influence remains insufficiently understood. In this study, we employ a three‐dimensional multi‐fluid magnetohydrodynamic (MHD) model, combined with MAVEN observations, to investigate the response of the dayside and near‐nightside ionosphere under varying solar EUV flux levels. The simulation results show that increased EUV flux leads to a significant rise in ion density, accompanied by a decrease in ion velocity across the terminator, which reflects the day‐to‐night ion flow. Since the increase in density is more significant, the overall ion transport flux across the terminator also increases with higher EUV flux. Analysis of the electric and magnetic fields reveals that the enhanced ion production under stronger EUV conditions results in a denser ionosphere and a corresponding reduction in electric field strength. In particular, both the Hall and motional electric fields, which dominate horizontal ion transport, tend to weaken with increasing ion density. This reduction in the electric field leads to slower trans‐terminator velocities. Additionally, the crustal magnetic field in the southern hemisphere modifies ion transport by deflecting and slowing the horizontal plasma flow from the dayside to the nightside.