The influence of sudden commencements on the rate of change of the surface horizontal magnetic field in the United Kingdom

Sudden commencements (SCs) are rapid increases in the northward component of the surface geomagnetic field, related to sharp increases in the dynamic pressure of the solar wind. Large rates of change of the geomagnetic field can induce damaging currents in ground power networks. In this work, the effect of SCs on the (one minute) rate of change of the surface magnetic field (R) at three UK stations is investigated. The distributions of R during SCs are shifted to higher values than the data set as a whole. Rates of change greater than 10 nTmin‐1 are 30‐100 times more likely during SCs, though less than 8% of the most extreme R (≥ 99.99th percentile) are observed during SCs. SCs may also precede geomagnetic storms, another potential source of large R. We find that the probability of observing large R is greatly enhanced for three days following an SC. In the 24 hours following an SC it is 10 times more likely than at any given time to observe rates of change between 10 and several hundred nTmin‐1. Additionally, between 90 and 94% of data (depending on station) above the 99.97th percentile is recorded within three days of an SC. All values of R ≥ 200 nTmin‐1 in the UK have been observed within three days of an SC. These results suggest that accurately predicting sudden commencements is critically important to identify intervals during which power networks at similar geomagnetic latitudes to the UK are at risk from large GICs.

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