Left (a)–(f): Superposed epoch analysis of magnetometer data from Inukjuak QC and EHD from the Tilly substation in the Hydro-Québec network (median values in bold black). Superposed epoch trigger is the negative B_z extremum for the 50 largest negative B_z impulsive events. Median SMU (red) and SML (blue) for these 2-h windows are shown in the bottom panel. Individual EHD traces are colorized in d, median values shown in e. Right (g)–(l): Inukjuak QC magnetometer readings for equivalent SME periods at the B_z extrema, and corresponding median EHD values at the Tilly substation. All SME values have been minimum EHD subtracted for each two-hour period. Events without HD data from Tilly were not used (5 of 50 events).