Anomalous Substorm Signatures During Sudden Solar-Wind Pressure Enhancements

Abstract

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. This paper examines two unique substorm events, occurring on 06 September 2017 and 19 December 2015, where both events involved minimal IMF Bz and prolonged negative By, alongside coincident pressure pulses. Despite these similar interplanetary conditions, the ground-based magnetic field observations revealed distinct differences in the location and intensity of the auroral electrojet. On 06 September, the electrojet was strongest in the post-midnight sector, whereas on 19 December, the maximum intensity occurred in the dawn sector. Simulations using the OpenGGCM magnetohydrodynamic model reveal that the substorm onset mechanisms differed between the two events. The 06 September event followed a typical IMF By-induced pattern with a shift in onset location to post-midnight, while the 19 December event exhibited an unusual onset, with plasma splitting and propagating sunward in both pre- and post-midnight sectors. These findings suggest that pressure pulses, rather than IMF By, are responsible for large shifts in substorm onset location. The study highlights the need for further investigation into multiple reconnection sites and the role of solar wind pressure in shaping substorm evolution.