Evaluating the use of geomagnetic indices for identifying potential damage to power grids

Extreme geomagnetic storms have the potential to have a damaging impact on power infrastructure, through the currents they induce in the ground, termed geomagnetically induced currents (GICs). Such events are often classified and forecast in terms of the Kp index, with a Kp=9o event described as a G5 storm on the NOAA Space Weather Scale for geomagnetic activity. However, this global average, mid-latitude, 3-hour index is unlikely to be the most useful indicator of the short time-scale variations in magnetic field that are of most concern to power transmission operators. For example, depending on latitude an observatory K index of nine can represent a magnetic variation from tens to a few thousand nT. We therefore investigate the relation between GICs, both measured and modelled, to a range of geomagnetic indices. We use a range of validation techniques to evaluate how well these indices perform at identifying periods of large GICs in the UK, focusing in particular on geomagnetic storms. We also extend the study to look at possibilities for a new local index, based directly on the rate-of-change of the magnetic field, as well as forecasts of indices.