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Variation of geomagnetic index empirical distribution and burst statistics across successive solar cycles.
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  • Aisling Bergin,
  • Sandra C Chapman,
  • Nicholas R. Moloney,
  • Nicholas Wynn Watkins
Aisling Bergin
University of Warwick

Corresponding Author:aisling.bergin@warwick.ac.uk

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Sandra C Chapman
University of Warwick
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Nicholas R. Moloney
London Mathematical Laboratory
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Nicholas Wynn Watkins
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The overall level of solar activity, and space weather response at earth, varies within and between successive solar cycles and can be characterized by the statistics of bursts, that is, time-series excursions above a threshold. We consider non-overlapping 1 year samples of the auroral electrojet index (AE) and the SuperMAG-based ring current index (SMR), across the last four solar cycles. These indices respectively characterize high latitude and equatorial geomagnetic disturbances. We suggest that average burst duration τ̅ and burst return period R̅ form an activity parameter, τ̅/R̅ which characterizes the fraction of time the magnetosphere spends, on average, in an active state for a given burst threshold. If the burst threshold takes a fixed value, τ̅/R̅ for SMR tracks sunspot number, while τ̅/R̅ for AE peaks in the solar cycle declining phase. Crossing theory directly relates τ̅/R̅ to the observed index value cumulative distribution function (cdf). For burst thresholds at fixed quantiles, we find that the probability density functions of τ̅ and R each collapse onto single empirical curves for AE at solar cycle minimum, maximum, and declining phase and for (-)SMR at solar maximum. Moreover, underlying empirical cdf tails of observed index values collapse onto common functional forms specific to each index and cycle phase when normalized to their first two moments. Together, these results offer operational support to quantifying space weather risk which requires understanding how return periods of events of a given size vary with solar cycle strength.
Jan 2022Published in Journal of Geophysical Research: Space Physics volume 127 issue 1. 10.1029/2021JA029986