Monday

The climate of space weather is modulated by the solar cycle. The overall level of solar activity, and the response at earth, varies within and between successive solar cycles. The auroral electrojet (AE) index characterises geomagnetic response at high latitudes, while the SuperMAG index, SMR, responds to the strength of the equatorial ring current. Bursts, or excursions above a threshold in an index time series, characterise space weather events. Quantifying space weather risk requires understanding how the return period of events of a given size varies with the strength of each solar cycle. The ratio of time series-averaged duration to burst return period may be interpreted as a dimensionless ‘activity parameter' which describes the fraction of time the magnetosphere spends, on average, in an active state. An important identity from crossing theory means that time series-averaged burst duration and burst return period are not independent quantities. For a given burst threshold, their ratio is equal to the complement of the underlying cumulative distribution function (CDF) of raw observations. We consider non-overlapping 1 year samples of AE and SMR at different solar cycle phases. We find that, for fixed value burst thresholds, the activity parameter follows the sunspot number double peak and, for AE, is peaked in the declining phase of the solar cycle. The tails of the underlying empirical CDFs of AE and SMR observations can be collapsed at different solar cycle phases when normalized to the first two moments. Taken together, these results offer operational support to quantifying the overall level of space weather activity.