Anomalous Substorm Signatures During Sudden Solar-Wind Pressure Enhancements

Magnetospheric substorms, characterized by the rapid release of energy stored in the magnetotail, play a central role in space weather dynamics. These events are typically triggered by enhanced magnetic reconnection between the Earth’s magnetic field and the interplanetary magnetic field (IMF). While substorms are often associated with southward IMF orientations, studies have also shown that they can occur even during northward IMF conditions, particularly when solar wind pressure pulses or strong IMF By components are present. In these cases, the location of substorm onsets and expansions can occur far from the usual midnight sector.

Notably, while the presence of an IMF By component tends to cause only small shifts in the local time sector of substorm onset (typically between ~22 MLT and ~01 MLT), solar wind pressure pulses can induce much larger shifts, potentially moving substorm onsets as far as the dawn or dusk sectors. This phenomenon suggests that ground-based observations of substorms centered around the dawn or dusk sectors could provide a unique signature of substorms triggered by pressure-induced impulses.

We provide examples of pressure-induced events exhibiting atypical onsets, with plasma splitting and propagating sunward in both pre-and post-midnight sectors. These findings suggest a likely cause for the large shifts in substorm onset locations during pressure pulses. The study highlights the need for further investigation into multiple reconnection sites and the role of solar wind pressure in shaping substorm evolution.