Global MHD modeling of ionospheric convection and field‐aligned currents associated with IMF B_y triggered theta auroras

Abstract

Using numerical magnetohydrodynamic simulations, we investigate the evolution of ionospheric convection and field-aligned currents (FACs) when θ auroras are formed in response to interplanetary magnetic field (IMF) B y transitions. When the polarity of IMF B y switches abruptly during northward IMF periods, the crossbar of the θ aurora is isolated from the flankside auroral oval and drifts into the polar cap. This drift motion is involved in a large round cell associated with new IMF B y , with sunward convection residing only on the dayside tip of the crossbar. There exists an IMF B y -controlled large-scale FAC system on the crossbar. When the θ aurora is drifting duskward (dawnward), the FACs are located on the dawnside (duskside) boundary of the crossbar adjacent to the "new" lobe. In contrast, the magnetospheric source region of the crossbar FAC system is located on the duskside (dawnside) boundary of the protruded plasma sheet adjacent to the "old" lobe. In the source region, plasma thermal pressure feeds the electromagnetic energy of FACs, and these processes can be interpreted as coupling of slow mode and Alfvén mode disturbances. In the ionosphere, the crossbar-associated FACs close with part of the region 1 currents associated with the new crescent cell. The magnetospheric source of that part of the region 1 FACs is located on the plasma sheet boundary and the magnetopause both adjacent to the new lobe. Dynamo processes in the old-lobe side and the new-lobe side work together to drive the ionospheric drift motion of the crossbar.

Generation of field‐aligned current (FAC) and convection through the formation of pressure regimes: Correction for the concept of Dungey's convection

“…Divergence of field‐perpendicular velocity should be realized along with the convergence of field‐parallel velocity. Although divergence of total velocity is not exactly zero, its magnitude is much smaller than divergence of field‐parallel velocity or divergence of field‐perpendicular velocity [ Watanabe et al , ].…”

“…Convergence of field‐perpendicular flow should occur along with the divergence of field‐parallel flow. These structures of pressure distribution and magnetic field structure along the flow path indicate that the slow mode expansion is driving the region 1 FAC [ Watanabe et al , ]. However, divergence‐convergence structure of field‐perpendicular flow is fading toward low altitude.…”

“…In a steady state and when the curvature of magnetic field is small, equation reduces to which is satisfied, and then convergence of field‐perpendicular flow generates the divergence of Poynting flux by compressing magnetic field along the convection path. Such structure is expected for the region 1 FAC [ Watanabe et al , ]. When curvature of magnetic field line is large, flow from the inside to the outside of the curved magnetic field increases its effect.…”

“…At the same time, convection acts to transport and deform magnetic field in accordance with the frozen‐in principle. In the transportation process, magnetic field must be distorted to specific structures at somewhere along the flow path so as to give the divergence of Poynting flux [ Watanabe et al , ]. From the induction equation for nonsteady state, the equation is obtained after some vector operations.…”

Cooperatives Roles of Dynamics and Topology in Generating the Magnetosphere‐Ionosphere Disturbances: Case of the Theta Aurora

“…Then, what occurs in the simulation results when the IMF changes? As is well known, if the IMF By is switched, the theta aurora is generated (Tanaka et al, ; Watanabe et al, ), and if the IMF Bz is switched, it becomes the substorm (Ebihara & Tanaka, , ; Tanaka, Ebihara, et al, ). In this paper, the case of By switch is presented.…”