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A clock for solar and geomagnetic activity
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  • Sandra C Chapman,
  • Scott W McIntosh,
  • Robert J. Leamon,
  • Nicholas Wynn Watkins
Sandra C Chapman
University of Warwick

Corresponding Author:s.c.chapman@warwick.ac.uk

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Scott W McIntosh
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Robert J. Leamon
University of Maryland
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Nicholas Wynn Watkins
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The frequency of major solar eruptions, and their space weather impacts at earth vary with the cycle of solar activity but large amplitude events can occur at any time. Each solar cycle has a distinct amplitude and duration so that the solar cycle dependent frequency of rare, extreme space weather events is challenging to quantify. By constructing the analytic signal of daily sunspot numbers since 1818 we construct a new solar cycle phase clock which maps each of the last 18 solar cycles onto a single time-base. This clock orders solar coronal activity and extremes of the aa index, which tracks geomagnetic storms at the earth’s surface over the last 14 solar cycles. We identify and quantify the occurrence times of a geomagnetically quiet solar cycle interval of ~4.4 years (~2 pi/5 phase or 40% of the cycle) in extent centered on solar minimum within which only two severe (aa>300nT) and one extreme (aa>500nT) geomagnetic storms occurred since 1868. The solar cycle modulation of activity is such that 1-3% of severe (aa>300nT) geomagnetic storms and 4-6% of C, M and X class solar flares occurred in the solar cycle quiet phase. Terminators of solar EUV bright point activity indicate the end of this quiet interval and the ‘switch on’ of increased frequency of solar flares and geomagnetic storms. This provides quantitative support to planning resilience against space weather impacts since only a few percent of all severe storms occur in this quiet interval and its start and end are forecast-able.