Giant GIC Undulations During the June 2015 Geomagnetic Storm Driven by Magnetosphere-Ionosphere Coupling Dynamics

Large Geomagnetically Induced Currents (GICs >10A) were recorded in two intervals during the 22-23 June 2015 geomagnetic storm in the north-eastern US GIC monitor stations. According to Faraday’s law of induction - an induced geoelectric field is produced by -dB/dt. For the June 2015 event, the Fredericksburg (FRD) magnetometer shows a corresponding change in B-field components during the intervals. Interval 1 - covering 19-20 UT on 22nd - shows 4 sinusoidal waves (+/-5A at an offset of ~10A). Interval 2 – spanning 03-06 UT on 23rd – shows giant undulations (up to +/-20 A and +/-50nT). Understanding GIC drivers, that is when and why they arise, is important to predict the stress on power systems. We employ wavelet analysis of timeseries and data fusion of observations from ground to space to gain a deeper understanding of the coupled Magnetosphere-Ionosphere system responsible for the undulations. Continuous wavelet transform provides the time-frequency distribution of the timeseries highlighting the key frequencies/periodicities. Data fusion provides a context for the drivers of the underlying key frequencies/periodicities. We find that Interval 1 GIC undulations were driven by severe compression of the magnetosphere and increased magnetic flux whereas Interval 2 had no apparent sudden upstream change – this implies inner magnetospheric source of ground B-field and GIC undulations. Data fusion of AMPERE-derived Field Aligned Currents (FACs), GNSS differential Total Electron Content (dTEC), and All Sky Imager (ASI) keograms for Interval 2 reveals quasi-periodic layered FACs and auroral intensifications alongside an ionospheric trough. These findings suggest the giant undulations were driven by nightside injection/BBF and the SubAuroral Polarization Stream (SAPS). MAGE (Multiscale Atmosphere Geospace Environment) simulation output provides additional insight into the coupling mechanisms that drive the undulations.