Using the aa Index Over the Last 14 Solar Cycles to Characterize Extreme Geomagnetic Activity
S. C. Chapman, R. B. Horne, N. W. Watkins
Abstract
Abstract 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.