Substorm Onset: Getting the Sequence of Events Right

New observations from THEMIS and complementary ground-based auroral observing programs have transformed our understanding of substorm onset. As far as we have come, however, there are still great challenges that must be addressed. Determining the relative timing of the different events which comprise onset is perhaps key amongst these, but has proven to be particularly challenging. Here we use optical observations of the onset in white light (THEMIS-ASI) and the 630 nm "redline" (TREx/REGO) to explore the causal relationship between events preceding onset (seen indirectly via auroral streamers) and onset (seen via auroral brightening). To do this, we employ a new analysis technique, called the 'circogram', which focuses on the direction of propagation of information in sequences of auroral images.