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Abstract:
A recent case study showed that shock impact angles control the generation of dB/dt variations on the ground during subsequently triggered substorms. More specifically, nearly frontal shocks tend to trigger fast and intense substorms associated with high-level dB/dt variations. On the other hand, highly inclined shocks tend to trigger slow and less intense substorms leading to low-level dB/dt variations. In this study, we use a ground magnetometer network with stations located in North America, from western Alaska to eastern Labrador (THEMIS/GMAG) and northwestern Greenland (Technical University of Denmark) to perform a statistical analysis of shock-related dB/dt variations during substorms with an updated shock list containing nearly 600 events. By using the well-known Spherical Elementary Current System (SEC) technique applied to ground magnetometer data, we construct geographical maps with dB/dt variations during the substorms and correlate these variations with shock impact angle as a function of MLATs and MLTs. We find that intense SECs and dB/dt variations occur in larger MLAT bandwidths for the more frontal shocks. By looking at specific MLTs associated with substorm activity, namely MLT > 21 hrs or MLT < 3 hrs around the magnetic midnight, we conclude that nearly frontal shocks indeed cause more intense dB/dt variations that cover wider geographic areas in comparison to highly inclined shocks. These results confirm previous observations of comparative studies and have profound implications to further space weather predictions and forecasting.
