Timing patterns in event lists: Recurrent geomagnetic storms

The time separations between events in an extended list of occurrence times may reveal recurrence patterns of predictive and interpretive value. A strategy for extracting the period and amplitude of cyclic or recurrent phenomena from lists of event times is developed and applied to 119 years of geomagnetic storm data. The ensemble of time intervals separating pairs of sudden commencement geomagnetic storms (SSCs) differs significantly from that expected for randomly occurring events, permitting the detection of preferred intervals between event occurrences and a determination of the strength and significance of recurrence patterns. Through 11 sunspot cycles, SSCs show persistent nonrandomness at the solar rotation period and its low multiples. The recurrence period is shorter on descending cycle phase than on the ascending phase. The strength of SSC recurrence near 27 days is comparable on ascending and descending phases, in contrast with the behavior of gradual commencement storms triggered by corotating solar wind streams.