Substorm Simulation: Insight Into the Mechanisms of Initial Brightening and Westward Traveling Surge

Abstract:

Auroral substorm expansion is often characterized by initial brightening of aurora, followed by a westward traveling surge (WTS). In spite of their attractive features, the underlying mechanisms of initial brightening and WTS are poorly understood. We propose a scenario for the formation of the initial brightening and WTS by analyzing the result obtained by a global magnetohydrodynamic (MHD) simulation: (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 when flow braking takes place. (3) Plasma moves earthward parallel to a field line. The plasma pressure is additionally enhanced at off-equator. Near-Earth dynamo is formed at the expanding front of the high-pressure region. (4) Flow vorticities are generated near the off-equatorial high-pressure region. When the resultant field-aligned current (FAC) is connected with the ionosphere, the upward FAC is rapidly increased in the ionosphere, which may manifest initial brightening. (5) Due to continued earthward flow, the high-plasma pressure region continues to expand. (6) The ionospheric conductivity continues to increase in the upward FAC region. (7) The gradient of the Hall conductivity gives rise to positive space charge (divergent electric field). (8) Due to the divergent electric field, the magnetospheric plasma moves counterclockwise at low altitude. (9) Localized upward FAC is generated at low altitudes, which may manifest WTS. The current line extending from the WTS is diverted from a magnetic field line by the strong perpendicular (diamagnetic current), and is connected with the mantle dynamo region by way of the near-Earth dynamo region. The near-Earth dynamo and the gradient of the Hall conductivity may be important in the formation of the upward FACs in substorm expansion phase.