Extreme Value Analysis of Induced Geoelectric Field in South Africa

Extreme geomagnetic disturbances occur rarely but can have great impact on technological systems such as power supply networks. Long‐term planning for extreme events requires the estimation of event impact for occurrence periods greater than the length of observed data. With this in mind an analysis of extreme geomagnetic events observed in South Africa (middle geomagnetic latitude) is performed over four solar cycles (1974–2015). An algorithm to identify active periods with minimum SYM‐H ≤−100 nT is demonstrated. The sum of induced electric field over the course of each event is used to characterize the severity of each active period. It is found that the severity index (accumulated electric field magnitude ΣE) shares a highly linear relationship with accumulated SYM‐H over each event. The index ΣE is lognormal distributed, with tail deviating greater than lognormal, confirming heavy‐tailed occurrence. A general Pareto distribution is fitted to the tail of the distribution and extrapolated to calculate the return levels of extreme events. Return levels of once in 100 and once in 200 year events are estimated to be 9.4 × 104 mV/km min and 1.09 × 105 mV/km min, respectively. The top three events, in ascending order of severity, are the March 1989 storm, the events of late October 2003, and the April 1994 event—a long interval of coronal‐hole driven disturbances, bookended by two intense geomagnetic storms. Plain Language Summary Planning for future extreme events like catastrophic floods or geomagnetic storms is difficult if the interval measured over is shorter than the planning period. This research deals with such a case. We only have 41 years' worth of data, but we wanted to know how extreme the once‐in‐a‐hundred‐years and once in 200 years events will be. To solve this, we quantify the severity of each significant event over the 41 year period by a single number, by adding up electric field induced in the Earth's surface by the geomagnetic perturbations over the course of an event. This collection of numbers—one for each event—is analyzed with statistical methods to guess how extreme the once in a 100 and once in 200 year events will be. The induced electric field tells us something about the effect that a geomagnetic storm will have on power network infrastructure, so this research enables power companies in South Africa to plan for future events. The methods used here are clearly explained so that this study can be reproduced by other researchers in