Why Do Sudden Commencements Sometimes Generate Disproportionate Geomagnetically Induced Currents?

Geomagnetically Induced Currents (GICs) in grounded, conducting infrastructure (e.g., power networks) represent an important space weather hazard. GICs are driven by the changing magnetic field at the Earth's surface. Due to a sparsity of GIC measurements, the rate of change of the horizontal geomagnetic field (Hʹ) is often used as a proxy. We focus on one cause of large GICs: Sudden Commencements (SCs) caused by an increase in solar wind dynamic pressure. Despite appearing homogeneous in 1‐min cadence magnetic field data, a systematic variation has been observed in the correlation with the resulting GIC, depending on the magnetic local time. We investigate two questions: (a) do the data capture SC morphology, and (b) are some SCs intrinsically linked to larger GICs? We find that 1‐min magnetic field measurements underestimate the maximum Hʹ, with systematic local time variation. We introduce an analytical model that describes the key components of an SC, the use of which strengthens the correlation between maximum Hʹ and GIC during SCs (r2 = 0.93). We conduct synthetic experiments with our analytical SC model and the GIC and geoelectric field models of Southern New Zealand and the United Kingdom, respectively. We find that the modeled GIC and geoelectric field linearly scale with the “size” of the SC, and non‐linearly with the “speed” of the magnetic change. Rotating the magnetic signature shifts the geoelectric field and GICs across the network. Forecasting the model parameters would enable robust forecasting of SC‐related GICs.