Characterization of Transient‐Large‐Amplitude Geomagnetic Perturbation Events

We present a characterization of transient‐large‐amplitude (TLA) geomagnetic disturbances that are relevant to geomagnetically induced currents (GIC). TLA events are defined as one or more short‐timescale (<60 s) dB/dt signature with magnitude ≥6 nT/s. The TLA events occurred at six stations of the Magnetometer Array for Cusp and Cleft Studies throughout 2015. A semi‐automated dB/dt search algorithm was developed to identify 38 TLA events in the ground magnetometer data. While TLA dB/dts do not drive GICs directly, we show that second‐timescale dB/dts often occur in relation to or within larger impulsive geomagnetic disturbances. Sudden commencements are not the main driver, rather the events are more likely to occur 30 min after a substorm onset or within a nighttime magnetic perturbation event. The characteristics of TLA events suggest localized ionospheric source currents that may play a key role in generating some extreme geomagnetic impulses that can lead to GICs.Plain Language SummarySevere space weather events like geomagnetic storms and substorms cause geomagnetically induced currents (GIC) in electrically conducting material on Earth that are capable of damaging transformers and causing large‐scale power grid failure. GICs are driven by large changes of the surface geomagnetic field, dB/dt, that have timescales of minutes to tens of minutes. Magnetic field variations with shorter‐timescales (<60 s) are not capable of driving large GICs directly, but we show here that they often occur in relation to or within larger storms, substorms and magnetic pulsation events that are capable of driving substantial GICs. In this study, we characterize these transient‐large‐amplitude (TLA) geomagnetic perturbation events and examine them in the context of other space weather events.Key PointsShort‐timescale (