Impact angle control of local intense d$B$/d$t$ variations during shock-induced substorms

TL;DR

This study shows that the impact angle of interplanetary shocks significantly influences the intensity and timing of geomagnetic disturbances during substorms, with nearly frontal impacts causing stronger and earlier effects.

Contribution

It provides a comparative analysis demonstrating how shock impact angles affect magnetospheric responses and geomagnetic variations during substorms, using multi-instrument data and modeling.

Findings

Nearly frontal shocks cause faster, more intense energetic particle injections.

Geographic regions with high dB/dt occur earlier and are larger in nearly frontal impact cases.

Auroral expansion is more rapid and pronounced with nearly head-on shock impacts.

Abstract

The impact of interplanetary shocks on the magnetosphere can trigger magnetic substorms that intensify auroral electrojet currents. These currents enhance ground magnetic field perturbations (d$B$/d$t$), which in turn generate geomagnetically induced currents (GICs) that can be detrimental to power transmission infrastructure. We perform a comparative study of d$B$/d$t$ variations in response to two similarly strong shocks, but with one being nearly frontal, and the other, highly inclined. Multi-instrument analyses by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Los Alamos National Laboratory spacecraft show that nightside substorm-time energetic particle injections are more intense and occur faster in the case of the nearly head-on impact. The same trend is observed in d$B$/d$t$ variations recorded by THEMIS ground magnetometers. THEMIS all-sky imager data show a fast and clear poleward auroral expansion in the first case, which does not clearly occur in the second case. Strong field-aligned currents computed with the spherical elementary current system (SECS) technique occur in both cases, but the current variations resulting from the inclined shock impact are weaker and slower compared to the nearly frontal case. SECS analyses also reveal that geographic areas with d$B$/d$t$ surpassing the thresholds 1.5 and 5 nT/s, usually linked to high-risk GICs, are larger and occur earlier due to the symmetric compression caused by the nearly head-on impact. These results, with profound space weather implications, suggest that shock impact angles affect the geospace driving conditions and the location and intensity of the subsequent d$B$/d$t$ variations during substorm activity.