Sudden Commencements and Geomagnetically Induced Currents in New Zealand: Correlations and Dependance

Changes in the Earth's geomagnetic field induce geoelectric fields in the solid Earth. These electric fields drive Geomagnetically Induced Currents (GICs) in grounded, conducting infrastructure. These GICs can damage or degrade equipment if they are sufficiently intense—understanding and forecasting them is of critical importance. One of the key magnetospheric phenomena are Sudden Commencements (SCs). To examine the potential impact of SCs we evaluate the correlation between the measured maximum GICs and rate of change of the magnetic field (H′) in 75 power grid transformers across New Zealand between 2001 and 2020. The maximum observed H′ and GIC correlate well, with correlation coefficients (r2) around 0.7. We investigate the gradient of the relationship between H′ and GIC, finding a hot spot close to Dunedin: where a given H′ will drive the largest relative current (0.5 A nT−1 min). We observe strong intralocation variability, with the gradients varying by a factor of two or more at adjacent transformers. We find that GICs are (on average) greater if they are related to: (a) Storm Sudden Commencements (SSCs; 27% larger than Sudden Impulses, SIs); (b) SCs while New Zealand is on the dayside of the Earth (27% larger than the nightside); and (c) SCs with a predominantly East‐West magnetic field change (14% larger than North‐South equivalents). These results are attributed to the geology of New Zealand and the geometry of the power network. We extrapolate to find that transformers near Dunedin would see 2000 A or more during a theoretical extreme SC (H′ = 4000 nT min−1). Plain Language Summary A changing magnetic field at the surface of the Earth will induce anomalous currents in conducting infrastructure, such as a power network. There are many processes that can cause the Earth's magnetic field to change, but we investigate one of the simplest: Sudden Commencements (SCs). SCs are caused by rapid increases in the density or velocity of the solar wind, and can be measured as a fast, mostly Northward change of the magnetic field on the ground. We compare the changes in the magnetic field with the currents observed at 75 locations across the New Zealand power network. We find a link between the changes in the magnetic field and the currents, but several locations appear to be more susceptible to large currents. We also find that some types of SC appear to cause larger currents and that the effect of SCs is greater on the sunlit side of the Earth. Finally, we