Generation mechanisms for magnetosphere‐ionosphere current systems deduced from a three‐dimensional MHD simulation of the solar wind‐magnetosphere‐ionosphere coupling processes

Abstract

Mechanisms that generate the field‐aligned current (FAC) systems in the magnetosphere‐ionosphere coupling scheme by virtue of the solar wind‐magnetosphere interaction are investigated with a three‐dimensional magnetohydrodynamic (MHD) simulation. As a simulation scheme, the finite volume total variation diminishing (TVD) scheme on an unstructured grid system is employed for precise calculations of the ionospheric region. In the ionosphere, the divergence of the Pedersen and Hall currents is matched with FAC, mainly assuming uniform conductivity. The present calculation reproduces the traditional region 1 and 2 currents in the polar ionosphere, for both the northward and southward interplanetary magnetic fields (IMFs). The calculated magnitude of the region 1 current becomes large on the dayside, in agreement with observational results. For the northward IMF, NBZ currents that dominate the entire polar cap are obtained, with a maximum on the dayside. This current is totally absent in the southward IMF result. Corresponding to the FACs, the northward IMF results in multicell convection in the polar ionosphere, and the southward IMF results in two‐cell convection. On the evening side, the calculated region 1 currents flow almost along the field lines away from the Earth toward the magnetospheric low‐latitude boundary layer (LLBL), then flow up the magnetopause across the field lines to high latitudes. The region 1 currents in the morning side are similar but opposite in direction. In the noon‐midnight meridian (xz) plane, the main part of the region 1 current passes the tailward side of the cusp in the magnetosphere. The region 1 current converges to a very narrow region in the noon‐midnight meridian (xz) plane when the IMF is northward, whereas it passes the noon‐midnight meridian (xz) plane diverging to wide regions in the x direction when the IMF is southward. These differences are attributed to the efficient current‐driving effect (J · E < 0) of the high‐latitude boundary layer (HLBL) for the southward IMF. The calculated region 2 currents on the evening (morning) side flow toward (away from) the Earth and close in the inner magnetosphere near the equator. The evening region 2 currents flow azimuthally from the inner boundary of the plasma sheet and show a sharp turn toward the Earth at the ring current region where strong drivers are distributed. For the northward IMF, the NBZ current that flows toward (away from) the evening (morning) polar cap ionosphere is connected with currents in the magnetotail. In the NBZ current loop, there is no remarkable driver or load (J · E > 0). In the evening magnetosphere, the NBZ current that flows into the dayside ionosphere passes the low‐latitude side of the NBZ current that flows into the nightside ionosphere, then it turns aside to the outward (+y) direction and turns back before reaching the dayside ionosphere. Consequently, the dayside NBZ current flows from the low‐latitude side of the lobe, while the nightside NBZ current flows from the high‐latitude side of the lobe. Calculation assuming nonuniform ionospheric conductivity results in a wedge‐current‐like structure in the evening side. This result indicates that the current generated in the ionosphere cannot be ignored in the magnetosphere‐ionosphere current systems.