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....

Formation of magnetospheric plasma population regimes coupled with the dynamo process in the convection system

“…The blue color also extends towards the mantle. It was already shown by Tanaka [1995Tanaka [ , 2000a and Siscoe et al [2000] that this blue region is the closure point of region-1 FAC. However, they did not realize a deep meaning of this region.…”

“…[11] By adopting the FV TVD scheme, Tanaka [1995] first reproduced the FAC numerically and clarified the 3-D structure of the FAC system showing that the region-1 FAC is closing over the high-latitude boundary layer and the region-2 FAC is closing primary in the low-latitude nightside plasma sheet. This success was the first step to discuss the self-consistent configuration of the M-I convection system.…”

“…In this real configuration, static balance can no longer be possible at the interfacing region. In this area the region-2 FAC system coupled to the convection must support sunward ÀrP force [Tanaka, 1995[Tanaka, , 2000a. As a consequence, the plasma sheet formation is also an inevitable result to maintain the self-consistency in the convection system.…”

“…Thus simulation studies become essential for such examinations. Recent MHD simulations have reproduced the magnetospheric closure process of FAC Tanaka, 1995;Crooker et al, 1998;Fedder et al, 1998;Siscoe et al, 2000] and revealed the relationship between convection and FAC, which has advanced our understanding of the interactions between plasma regimes, current system, and convection.…”

Configuration of the magnetosphere‐ionosphere convection system under northward IMF conditions with nonzero IMF By

“…In other words, the magnetospheric convection is driven mechanically through the transfer of solar wind plasma onto the geomagnetic field [Johnson, 1978;. Then the magnetospheric convection is mapped down to the ionosphere through the exchange of field-aligned current (FAC) Tanaka, 1995]. The earliest model of Dungey [1961] and later reproductions by many authors showed that the magnetospheric convection under a southward IMF condition consists of the bow reconnection, subsequent tailward transport of open field lines, and the reconnection at the downstream.…”