A new technique for determining Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE)

Abstract

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.

What effect do substorms have on the content of the radiation belts?

“…Forsyth et al . [] found that the expansion phase onsets determined using the 75th percentile gave good agreement with existing auroral‐based [ Frey and Mende , ; Liou et al ., ] and magnetometer‐based [ Newell and Gjerloev , ] substorm onset lists. Thus, we use this percentile for all phase identifications in this study.…”

Plasma Sheet Pressure Variations in the Near‐Earth Magnetotail During Substorm Growth Phase: THEMIS Observations

“…Here we use the criterion that the standard deviation of D p during the 90 min (60 min preceding and 30 min following) is smaller than 30% of the average D p . The second step is to further select the isolated substorm events from the IMF southward turning events. The SuperMAG SML index (Gjerloev, ) and substorm onset lists from Newell and Gjerloev () and Forsyth et al () (a specified expansion phase threshold of 50%) are employed in the selection. The preceding period should be with average value of SML greater than −100 nT in 1 h, and there should be no substorm onsets listed by Newell and Gjerloev () and Forsyth et al ().…”

Systems science of the magnetosphere: Creating indices of substorm activity, of the substorm‐injected electron population, and of the electron radiation belt

“…In Yakymenko and Borovsky [] this SML‐jump method of identifying substorm onsets was compared with other methods: basically, the SML‐jump method identifies large, nontransient events and the occurrence statistics of these events agrees with the occurrence statistics of substorms identified by energetic‐particle injections into geosynchronous orbit [e.g., Borovsky et al , ; Borovsky and Yakymenko , ] and with prior schemes that identified substorms with jumps in the AL index [e.g., Prichard et al , ]. The SML‐jump events are statistically different from the events chosen by the Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE) algorithm [ Forsyth et al , ] and the events denoted as MPB events [ Chu et al , ], with the SOPHIE and MPB events being smaller and much more numerous [ Yakymenko and Borovsky , ].…”

“…Second, there is not an agreed‐upon method of determining the onset time of a substorm [ Kepko et al , ; Nishimura et al , ; Lui , ; Borovsky and Yakymenko , ]. Third, there is confusion about substorm onsets versus substorm activations (reactivations and intensifications) [ Kisabeth and Rostoker , ; Forsyth et al , ; McPherron and Chu , ; Yakymenko and Borovsky , ]. Fourth, whatever the data source for creating a substorm index, a continuous data coverage is necessary.…”