  We present a new quantitative technique that determines the times and
durations of substorm expansion and recovery phases and possible growth phases
based on percentiles of the rate of change of auroral electrojet indices. By
being able to prescribe different percentile values, we can determine the onset
and duration of substorm phases for smaller or larger variations of the auroral
index or indeed any auroral zone ground-based magnetometer data. We apply this
technique to the SuperMAG AL (SML) index and compare our expansion phase onset
times with previous lists of substorm onsets. We find that more than 50% of
events in previous lists occur within 20 min of our identified onsets. We also
present a comparison of superposed epoch analyses of SML based on our onsets
identified by our technique and existing onset lists and find that the general
characteristics of the substorm bay are comparable. By prescribing user-defined
thresholds, this automated, quantitative technique represents an improvement
over any visual identification of substorm onsets or indeed any fixed threshold
method.
