Why do Some Sudden Commencements Generate “Disproportionate” Geomagnetically Induced Currents?

A key space weather hazard is the generation of Geomagnetically Induced Currents (GICs) in grounded, conducting infrastructure (e.g., power networks).  These GICs are driven by the changing magnetic field at the surface of the Earth and in extreme cases can cause disruption or even damage to power systems.  Due to a sparsity of GIC measurements around the globe, the rate of change of the magnetic field (e.g., H’) is often used as a proxy, under the assumption that larger rates of change of the geomagnetic field will be related to larger GICs.  While a range of magnetospheric processes can result in large GICs, in this work we focus on one: Sudden Commencements (SCs).  SCs are rapid, coherent changes in the geomagnetic field caused by the impact of a large increase in solar wind dynamic pressure (e.g., an interplanetary shock).  Globally, in one-minute cadence ground magnetic field data SCs appear relatively homogenous, lasting only a few data points.  However, it has previously been found that in New Zealand SCs on the dayside have been linked to 30% larger measured GICs for a given H’.  We investigate a deceptively simple question: why?

In this work we examine the sub-minute structure of SCs in New Zealand and their impact on the resulting GICs.  We introduce an analytical model that describes the key features of the magnetic field signature, allowing us to fully describe the key features of an SC.  The use of parameters (e.g., maximum H’) from the fitted analytical model strengthens the correlation between maximum H’ and GIC during SCs, but leaves remnant dependencies which are yet to be explained.  We conduct synthetic experiments with our analytical SC model and a high resolution magnetotelluric-derived map of the southern part of New Zealand to examine which properties of an SC make it more-likely to cause disproportionately large GIC.