Characterization of Transient-Large-Amplitude Geomagnetic Perturbation Events

We present a characterization of transient-large-amplitude (TLA) geomagnetic disturbancesthat are relevant to geomagnetically induced currents (GIC). TLA events are defined as one or moreshort-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.