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\begin{document}

\title{Using the aa index over the last 14 solar cycles to characterize extreme
geomagnetic activity}



\author[1,1]{Sandra C Chapman}%
\author[2,2]{Richard B. Horne}%
\author[3,3]{Nicholas Wynn Watkins}%
\affil[1]{University of Warwick}%
\affil[2]{British Antarctic Survey}%
\affil[3]{LSE}%


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  \date{May 25, 2024}


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\begin{abstract}
Geomagnetic indices are routinely used to characterize space weather
event intensity. The DST index is well resolved, but only available over
5 solar cycles. The aa index extends over 14 cycles but is highly
discretized with poorly resolved extremes. We parameterize extreme aa
activity by the annual averaged top few \% of observed values, show
these are exponentially distributed and they track annual DST index
minima. This gives a 14 cycle average \textasciitilde{}4\% chance of at
least one great (DST\textless{}-500nT) storm and \textasciitilde{}28\%
chance of at least one severe (DST\textless{}-250nT) storm per year. At
least one DST=-809{[}-663,-955{]}nT event in a given year would be a
1:151 year event. Carrington event estimate DST\textasciitilde{}-850nT
is within the same distribution as other extreme activity seen in aa
since 1868 so that its likelihood can be deduced from that of more
moderate events. Events with DST\textless{}-1000nT are in a distinct
class, requiring special conditions.%
\end{abstract}%



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