Operations to Research: The Power Grid Comes On Line in Support of GIC Monitoring

The first ‘Strong’ geomagnetic storm of solar cycle 25 on May 12 May 2021 has been associated with solar filament material interacting with a coronal hole and possibly the heliospheric current sheet during the transit to Earth. New ground-based monitors within the North American Electric Reliability Cooperation (NERC) system observed the Geomagnetically-Induced-Current (GIC) response to the train of structures passing Earth. The shock and sheath ahead of the solar wind transients produced moderate signals in ground based GIC detectors and ground magnetometers. A particular solar wind structure moving at ambient speed in the middle of the train triggered a significant GIC spike and a coherent, anomalously- long GIC response across a large portion of the North American continent. Ground-based indices presented mixed signals during the storm (AE index ~ 1500 nT, Kp =7, Dst ~ -60 nT). There are indications, based on the strong southward interplanetary magnetic field at -20 nT, but weak Dst, that geospace was ‘weakly shielded’ during the interaction. We review the unusual combination of solar wind structures and their interactions with geospace and the ground with particular focus on the GIC-driving structure. Newly available NERC data helped untangle the story behind this stealthy GIC event. We anticipate this new era of in-situ GIC data will provide additional insight to GIC- and space-weather science in the years ahead.