Abstract
AbstractGeomagnetic 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.
Highlights
Extreme space weather events significantly disrupt systems for power distribution, aviation, communication, and satellites; they are driven by large-scale plasma structures emitted from the solar corona, but their impact depends on a variety of factors (Baker & Lanzerotti, 2016)
The aa index is by construction based on combining observations that are logarithmically discretized in amplitude, and individual records of the 3 hr aa index will have uncertainties that are both significant and nontrivial to estimate (Bubenik & Fraser-Smith, 1977)
The aa index is constructed from observations that are logarithmically discretized in amplitude and individual records of the 3 hr aa index will have uncertainties that are both significant and nontrivial to estimate (Bubenik & Fraser-Smith, 1977); in particular, its extreme excursions are not well resolved in amplitude
Summary
Extreme space weather events significantly disrupt systems for power distribution, aviation, communication, and satellites; they are driven by large-scale plasma structures emitted from the solar corona, but their impact depends on a variety of factors (Baker & Lanzerotti, 2016). We use this linear mapping to convert these 150 annual averages of the top few percent of aa values into proxy DST extremes This gives us 150 estimates for the annual minimum DST that occurred over the last 14 solar cycles of activity. We compare these estimates with the range of minimum DST for a 1:151 event inferred from the 14 solar cycle proxy DST extremes record This provides an assessment of whether the Carrington event was a more intense version of the other superstorms that have occurred since 1868 or whether it was in a class of its own, which would require the concurrence of special conditions in the corona and solar wind and at the Earth. If it is the former can we use the set of observed storms to try to predict how likely such an event is in the future
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