Geomagnetically induced currents or GICs are signatures of a rapidly time-varying magnetic field (dB/dt) and occur mainly during substorms and storms. When, where and why exactly GICs may occur, is still vague. Thus, we investigated storms for the last 40 years (from 1980 with a storm-list created by W.T. Walach) and analyzed the negative and positive dB/dt spikes (threshold of 500 nT/min) in the north and east component using a worldwide coverage (SuperMAG).
Our analysis confirmed the existence of two dB/dt spikes “hotspots” located in the pre-midnight and in the morning MLT sector, independently of the geographic location of the stations. The associated physical ionospheric phenomena are most probably substorm current wedge (SCW) onsets and westward travelling surges (WTS) in the evening sector, and wave- or vortex-like current flows in Omega bands in the morning sector.
Additionally, we observed a spatio-temporal evolution of the negative northern dB/dt spikes. The spikes initially occur in the pre-midnight sector, and then develop in time towards the morning sector. This spatio-temporal sequence is correlated with bursts in the AE index, and can be repeated several times throughout a storm. Finally, we investigated the intensity (Dst and AE) of the storms compared to the number of dB/dt spikes, but we did not find any correlation. This result implies that moderate storm with many spikes can be as (or more) dangerous for ground-based infrastructures than a major storm with fewer dB/dt spikes.
Our findings may help to improve the GICs forecast to accurately predict dB/dt spikes.