Cause of Shock‐Associated Substorm Activity on 24 September 1998: A Global MHD Simulation Study

Abstract We investigated the shock‐triggered substorm activity that occurred on 24 September 1998. Results from a global magnetohydrodynamic simulation show that the interplanetary shock immediately triggered substorm activity characterized by a sharp intensification of the auroral electrojets and widespread auroral brightening, which are consistent with observations. In the simulation, the following processes occur: When the interplanetary shock struck the magnetopause, compressional waves were excited. As the compressional waves propagated antisunward, the plasma pressure increased in the magnetosphere, leading to enhanced ionospheric conductivity. Almost simultaneously, field‐aligned currents (FACs) were intensified, particularly on the dayside, resulting in stronger ionospheric convection. The combination of enhanced conductivity and convection rapidly intensified the auroral electrojet. During this interval, the z ‐component of the interplanetary magnetic field was nearly zero, while the y ‐component was largely enhanced, producing a substantial increase in the convection. In contrast to classical substorms, several typical features were absent, including auroral bulges or surges, localized enhancement of FACs near midnight, and clear dipolarization in the near‐Earth tail. Despite these differences, the shock‐triggered substorm activity may represent a distinct subclass of substorms, driven primarily by direct solar wind forcing. Notably, magnetic reconnection in the near‐Earth tail had already occurred prior to the shock arrival, but we could not identify a clear role of the reconnection in the subsequent substorm activity. Of course, the possibility of unloading processes cannot be entirely ruled out. The nature of shock‐triggered substorm activity remains open to discussion, and further detailed observational studies are needed to clarify this issue.