Statistical properties of substorms during different storm and solar cycle phases

Abstract. Substorm properties during different storm phases have been studied using an automated recognition of substorm and storm phases in the auroral electrojet (AL) and ring current (Dst) index data from 1995–2009. The large number of events (about 500 storms and 15 000 substorms) provides statistically reliable distributions, average behaviour and long time series of simple parameters, such as durations and intensities. The phases of storms and substorms have been examined independently. Substorm phases have been further combined to single and multi-cycle events. The former consist of one growth, one expansion and one recovery phase, while the latter include multiple expansion and recovery phases after one growth phase.

Our findings show that most substorms take place during non-storm times, and substorms during storm initial phases resemble isolated non-storm time substorms. Both during storm initial phases and non-storm times, the substorm growth phases may last longer than the other substorm phases. Substorm recovery phase is typically the longest phase but its duration also varies most. The longest substorm recovery phase duration was observed during multi-cycle substorms. The longest substorm expansion and storm main phases were found during the years close to the solar maximum. The shortest substorm events (the shortest phase durations) are the single-cycle substorms. The period of expansion onsets during multi-cycle substorms varied hugely for events with a small number of expansion phases. For events with a larger number of expansions, a clearer periodicity of about one hour (median value) was suggested.