Large, Long‐Lasting Mid‐Latitude Geomagnetically Induced Currents During a Moderate Geomagnetic Storm

Geomagnetically Induced Currents (GICs) are a key space weather hazard to ground-based infrastructure, and can cause mis-operation or even equipment failure. The solar wind interacts with the Earth's geomagnetic field, causing the magnetic field at ground level to vary with time. This variability causes GICs in grounded, conducting networks via coupling with the subsurface geology. Most studies of the hazard focus on short, high amplitude GICs, but long intervals of lower amplitude GIC can also be problematic, representing a rare but known pathway to equipment damage and power failure. We evaluate an interval in June 2015 where the power network in New Zealand experienced long-duration, steadily increasing GICs across the South Island. These GICs were up to 20A and lasted approximately 90 min during an otherwise moderate storm interval, while New Zealand was in the dawn sector. We investigate the magnetospheric cause, attributing them to a possible diversion of the ring current into the ionosphere. We test the ability of modeling and proxies to capture this hazard, finding that it can be recreated, but is dependent upon the regional geology, and requires detailed knowledge of the network configuration. We model two hypothetical examples, (a) where the magnetic perturbation is larger and (b) where a different mid-latitude location is exposed, finding that larger currents are possible and that the vulnerability is not unique to New Zealand. Our case study highlights the limitations of geomagnetic indices, which we show to be vulnerable to influence from such rare mid-latitude phenomena.