Large, Long-lasting Mid-Latitude Geomagnetically Induced Currents During a Moderate Geomagnetic Storm

Large Geomagnetically Induced Currents (GICs) are a key space weather hazard to ground-based infrastructure. These GICs act as a DC offset in typically AC power networks and when very large can cause mis-operation or even equipment failure. The dynamic solar wind interacts with the Earth’s geomagnetic field, causing the magnetic field as measured on the ground to vary with time. Via coupling with the geology of the solid Earth, this variability causes GICs to be created in grounded conducting networks. Most studies of the GIC hazard focus on short, fast changes of the geomagnetic field, for example large “spikes” in the one-minute rate of change of the horizontal geomagnetic field.However, recent work has reminded researchers that long intervals of lower (relative) GIC can also be problematic, and represents a known pathway to equipment damage and long-lasting power failure. In this study we evaluate a curious interval where the power network in New Zealand experienced long duration, steadily increasing GICs across the South Island. These GICs were up to 20 A and lasted approximately 90 minutes, manifesting in the mid-latitude dawn sector during an otherwise moderate geomagnetic storm. We investigate the magnetospheric cause of the ground observations, and the ability of contemporary modelling techniques to capture this facet of the GIC hazard to ground-based infrastructure. Further, this case study highlights the limitations of geomagnetic indices, which we show to be vulnerable to contamination from such rare mid-latitude phenomena.