Impact Angle Control of Local Intense dB/dt Variations During Shock‐Induced Substorms

The impact of interplanetary shocks on the magnetosphere can trigger magnetic substorms that intensify auroral electrojet currents. These currents enhance ground magnetic field perturbations (dB/dt), which in turn generate geomagnetically induced currents (GICs) that can be detrimental to power transmission infrastructure. We perform a comparative study of dB/dt 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 dB/dt 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 dB/dt 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 dB/dt variations during substorm activity. Plain Language Summary Solar perturbations propagating in the interplanetary (IP) space can cause significant geomagnetic activity when they impact Earth. Such magnetic disturbances occur in the geospace and on the ground, manifested as, for example, satellite surface charging and undesirable geoelectric currents flowing in large‐scale power transmission lines. In this study, we compare the effects caused by two similarly strong solar perturbations that impacted Earth with two very distinct orientations: one nearly head‐on and the other highly inclined. We use an extensive list of data sets covering observations in the IP space, in the geospace, and on the ground. We find that magnetic field perturbations and auroral brightening are much more intense in the nearly frontal