Storm sudden commencement events and the associated geomagnetically induced current risks to ground‐based systems at low‐latitude and midlatitude locations

Large impulsive geomagnetic field disturbances from auroral current systems have always been well understood as a concern for power grids in close proximity to these disturbance regions, predominantly at high‐latitude locations. Magnetospheric shocks (SSCs) due to large‐scale interplanetary pressure pulses are familiar from a geomagnetic disturbance perspective but have not been understood in the context as a potential driver for large geomagnetically induced currents (GICs). Observational evidence and analysis contained in this paper illustrate such events are capable of producing large geoelectric fields and associated GIC risks at any latitude, even equatorial locations. A large SSC disturbance on 24 March 1991 produced some of the largest GICs ever measured in the United States at midlatitude locations. The analysis methods and understanding of electromagnetic coupling processes at that time were unable to fully explain these observations. Electrojet‐driven disturbances common at high‐latitude locations during geomagnetic substorms cause large amplitude variations in locally observed B field, while SSC events are characterized as low‐amplitude B field disturbance events. Disturbance amplitude only accounts for part of the electromagnetic coupling process. The attribute of spectral content of the disturbance is equally important and heretofore had not been well understood and was not well measured unless high‐cadence observations were conducted. The deep‐earth ground conductivity also provides an important enabling role at higher frequencies. Deep‐earth ground response to geomagnetic field disturbances is highly frequency‐dependent. Therefore for nearly all ground conditions the higher the spectral content of the incident magnetic field disturbance, the higher the relative geoelectric field response.