Using the Index Over the Last 14 Solar Cycles to Characterize Extreme Geomagnetic Activity

Geomagnetic indices are routinely used to characterize space weather event intensity. The index is well resolved but is only available over five solar cycles. The index extends over 14 cycles but is highly discretized with poorly resolved extremes. We parameterize extreme activity by the annual‐averaged top few percent of observed values, show that these are exponentially distributed, and they track annual index minima. This gives a 14‐cycle average of 4% chance of at least one great ( nT) storm and 28% chance of at least one severe ( nT) storm per year. At least one nT event in a given year would be a 1:151 year event. Carrington event estimate nT is within the same distribution as other extreme activity seen in since 1868 so that its likelihood can be deduced from that of more moderate events. Events with nT are in a distinct class, requiring special conditions. Here we use measurements of disturbances in the Earth's magnetic field that go back to 1868, and we present a novel way of analyzing the data to identify the largest magnetic storms going back some 80 years longer than has been done before. As a result, we are able to state the chance of at least one superstorm occurring in a year. We find that on average there is a 4% (28%) chance of at least one great (severe) storm per year and a 0.7% chance of a Carrington class storm per year, which can be used for planning the level of mitigation needed to protect critical national infrastructure. We present a new method that parameterizes extremes of 14 solar cycles of the geomagnetic index We find a 4% (28%) chance of at least one great (severe) storm per year over 14 solar cycles A perturbation weaker than 1,000 nT Carrington storm is in the same occurrence rate distribution as other superstorms since 1868