Asymmetrical Ion Dynamics and Its Impact on Plasma Boundaries in the Martian Magnetosphere

Abstract

The asymmetry in plasma flow governed by the direction of solar wind motional electric field $E_{\text{SW}}=-V_{\text{SW}}\times B_{\text{IMF}}$, which can be referred to as $E_{\text{SW}}$ asymmetry, is one of the most important asymmetries in the Martian plasma environment. In this study, a multifluid magnetohydrodynamic (MHD) model is employed to investigate the $E_{\text{SW}}$ asymmetry of ion motions on the magnetic pileup boundary (MPB) and inside the magnetosphere. The simulation results indicate a more intense solar wind penetration across the bow shock in the ${+Z}_{\text{MSE}}$ hemisphere, which enhances the pileup of magnetic field lines through the mass loading process and compresses the MPB. The electric fields, especially the motional electric field $E_M$, exhibit apparent $E_{\text{SW}}$ asymmetries, accelerating solar wind in the ${-Z}_{\text{MSE}}$ hemisphere while decelerating protons in the ${+Z}_{\text{MSE}}$ hemisphere. For planetary ions, the motional electric field accelerates ions upward in the ${+Z}_{\text{MSE}}$ hemisphere while hindering the ion outflow in the ${-Z}_{\text{MSE}}$ hemisphere, resulting in the energetic ion plume. In the ${+Z}_{\text{MSE}}$ hemisphere, planetary ions also experience more intense horizontal accelerations exerted by the electric fields, with the horizontal transport direction mainly controlled by $E_M$ direction. North-South asymmetries exist in the morphology of electric fields and solar wind deflection, which is primarily due to the crustal field. The Hall electric field and ambipolar electric field are higher in the southern hemisphere, applying a strong deflection to the solar wind. By comparison, $E_{\text{SW}}$ asymmetry is more significant in affecting proton motions near the MPB, while the MPB asymmetry is dominated by the crustal field.