Abstract
AbstractWe 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.
Highlights
Substorms are the elemental dissipative events in the coupled solar wind-magnetosphere-ionosphere system that process ~1015 J of captured solar wind energy during their lifetime [Tanskanen et al, 2002]
Substorm expansion phases are accompanied by a brightening and expansion of the nightside aurora [Akasofu, 1964], electromagnetic ULF waves and an enhancement in the auroral electrojet current due to the cross-tail current diversion, which results in a deflection of the magnetic field at ground level
Unlike the most commonly used onset lists, SOPHIE is not limited to identifying expansion phase onset; we identify the start of the possible growth phases and recovery phases and the durations of all substorm phases
Summary
Substorms are the elemental dissipative events in the coupled solar wind-magnetosphere-ionosphere system that process ~1015 J of captured solar wind energy during their lifetime [Tanskanen et al, 2002]. Substorm expansion phases are accompanied by a brightening and expansion of the nightside aurora [Akasofu, 1964], electromagnetic ULF waves (for a review, see Rae, I.J. and Watt, C.E.J., ULF waves above the nightside auroral oval during substorm onset, accepted in AGU Geophysical Monograph Series) and an enhancement in the auroral electrojet current due to the cross-tail current diversion, which results in a deflection of the magnetic field at ground level. First identified by McPherron [1970] and which, on average, lasts 30–90 min [Li et al, 2013], magnetic flux is added to the magnetotail lobes through reconnection at the dayside magnetopause This process enhances magnetospheric convection [Axford, 1969] and the ionospheric electrojets, resulting in a small deflection of the H component of the ground magnetic field at auroral latitudes [McPherron, 1970]. While isolated substorms usually follow this growth-expansion-recovery paradigm, events with multiple onsets or intensifications, seen as expansion phases occurring immediately following a recovery phase, are reported [Pytte et al, 1976]
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