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....

[1] The substorm mechanism is investigated by analyzing the numerical results obtained through the use of the recently refined magnetohydrodynamic model. After showing the growth phase features, the numerical solution reproduces the observed signatures of a substorm onset, including the formation of a near-Earth neutral line (NENL), earthward directed flow in the plasma sheet, a dipolarization, a geosynchronous D deflection, the development of the nightside field aligned currents (FACs), and electrojets in the ionosphere. The onset is triggered by a sudden collapse of the plasma sheet and a successive formation of a high-pressure region in the inner magnetosphere. The energy source of this high-pressure region is the magnetic tension released from the NENL. The tail disturbance is primary transmitted to the ionosphere by the region 2 FAC accompanying the high-pressure region formed in the inner magnetosphere. The associated nightside region 1 FAC is not connected to the traditional current wedge but to the cusp region. The region 1 FAC path to the ionosphere develops from dayside to nightside, so as to construct a grand loop with the region 2 FAC from the partial ring current. In the grand loop, the region 1 FAC provides a short circuit in the ionosphere for the region 2 FAC. In the expansion phase, convection enhances to relax the distorted pressure distribution formed at the onset, accompanied by a further development of dipolarization and a thickening of the plasma sheet in the magnetosphere and increases in the westward and eastward electrojets in the ionosphere. The present model can explain the explosive growth phase and the thinning of the midtail plasma sheet just after the onset.

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[1] The magnetospheric response to a solar wind impulse, a geomagnetic sudden commencement, is studied using an MHD model of the coupled solar windmagnetosphere-ionosphere system. This paper discusses propagation of the first signal launched by the impulse and generation of the field-aligned current that causes the ground magnetic signal detected as the preliminary impulse (PI). It is revealed that the PI current is first excited as an enhanced Chapman-Ferraro current in the magnetopause and next turns to the magnetosphere along the wavefront of the compressional signal launched by the impulse. It is finally converted to a field-aligned current via mode coupling due to plasma nonuniformity. The current in the wavefront region is an inertia current. We present a quantitative model of the PI model presented by Araki [1994] by using a numerical simulation.

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[1] A geomagnetic sudden commencement (SC) is studied numerically based on a model of buffeting the magnetosphere by a solar wind density impulse. This paper treats two successive current systems in the main impulse (MI) phase. The two current systems have different current generating mechanisms. The first current generator appears behind the wavefront of a compressional disturbance launched by the impulse. The inertia current of the compressional mode is generated by free energy due to deceleration of plasma flows. A field-aligned current (FAC) is excited through mode conversion from the compressional wave in a V A gradient region. The magnetospheric flows and the ionospheric flows are not connected self-consistently to each other. The second generator is located in the tailward side of the cusp. It is the same as the generator of the region 1 current system. The current generated there is connected with the FAC with the region 1 sense via a diamagnetic current flowing around an isolated enhancement of pressure in the nightside equatorial magnetosphere. The pressure enhancement is induced through compression of the magnetospheric flank due to the solar wind impulse. In this period, plasma convection vortices appear both in the magnetosphere and in the ionosphere, which are correspondent to each other. This is a peculiar convection confined within the magnetosphere (the SC transient cell convection). This convection is driven though compression of the magnetospheric flank due to the solar wind impulse.

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Abstract. The substorm mechanism is investigated from a resistive magnetohydrodynamic simulation under the assumption that the magnetotail becomes more diffusive as it goes further downtail. The simulation uses the finite volume total-variation diminishing scheme on an unstructured grid system to evaluate the magnetosphere-ionosphere coupling effect more precisely and to reduce the numerical viscosity in the near-Earth plasma sheet.