Substorm simulation: Formation of westward traveling surge

Auroral substorm expansion is characterized by initial brightening of aurora, followed by a bulge expanding in all directions, and a westward traveling surge (WTS). On the basis of the result obtained by a global magnetohydrodynamic simulation, we propose a scenario for the onset and the subsequent formation of WTS. (1) Near‐Earth neutral line releases magnetic tension in the near‐Earth plasma sheet to compress plasma and accelerate it earthward. (2) Earthward, perpendicular flow is converted to parallel flow in the near‐Earth tail region. (3) Plasma moves earthward parallel to a field line. The plasma pressure is additionally enhanced at off‐equator with an expanding slow‐mode variation. (4) Flow vorticities coexist near the off‐equatorial high‐pressure region. Resultant field‐aligned current (FAC) is connected to the ionosphere, which may manifest initial brightening. (5) Due to continued earthward flow, the high‐plasma pressure region continues to expand to the east and west. (6) The ionospheric conductivity continues to increase in the upward FAC region, and the conductivity gradient becomes steeper. (7) The convergence of the Hall current gives rise to divergent electric field near the steep gradient of the conductivity. (8) Due to the divergent electric field, magnetospheric plasma moves counterclockwise at low altitude (in the Northern Hemisphere). (9) The additional flow vorticity generates a localized upward FAC at low altitudes, which may manifest WTS, and redistributes the ionospheric current and conductivity. Thus, WTS may be maintained in a self‐consistent manner, and be a natural consequence of the overflow of the Hall current.