Triggering of substorm identified using modern optical versus geosynchronous particle data

There has been a long debate as to the extent to which substorms are externally triggered by the IMF. Previous works on substorm triggering have shown that more than 50% of the substorms are triggered by the northward turning of the IMF Bz. However, recent studies indicated a much lower percentage. Now we have the capability to study substorm onsets with the high-resolution all-sky imager (ASI) array of the THEMIS program, obtaining onsets unambiguously, including for very weak substorms, which are difficult to identify with the AL index. We examine triggering using three different onset lists: The THEMIS ASIs list, IMAGE-FUV substorm list, and events with large geosynchronous injections, which consist of substorms with increasingly large average intensity. These onset lists are analyzed using three external triggering criteria: (1) a criteria based on Lyons et al. [1997] criteria; (2) a relaxation of the Lyons et al. criteria based on the visual criteria proposed by Hsu et al. [2003]; and (3) a further relaxation of the Lyons et al. criteria, requiring the same conditions proposed in the visual criteria by Hsu et al. but without the growth phase southward IMF requirements. The triggering percentage for the events with large injections is higher than the triggering percentage for the other onset lists and reaches up to 60% applying the relaxed criteria. This result suggest that may be triggers are more easily identified, or that triggering is more common, for larger than for smaller substorm events. Our analysis does not allow us to ascertain whether or not the relaxation of the criteria indicates that the real triggering percentage is larger than given by the Lyons et al. criteria, but this suggests that this is a possibility that should be considered, particularly because the Lyons et al. criteria were based on very few events.