Relations between multiple auroral streamers, pre‐onset thin arc formation, and substorm auroral onset

[1] Recent ground‐based imager observations have provided evidence of precursor auroral activity leading to substorm auroral onset, where the precursor is initiated by a poleward boundary intensification (PBI) followed by an auroral streamer moving equatorward toward the onset latitude leading to substorm auroral onset. However, since many streamers do not lead to substorms, the question arises as to what conditions are required for streamers to lead to onset. Using 382 events detected by the THEMIS all‐sky imagers during 2007–2009, we examined the properties of latitudinally thin, quiet arcs that eventually break up during the substorm auroral onset and the relationship of such quiet arcs to streamers. We found that a pre‐existing latitudinally thin quiet arc that leads to auroral onset is much brighter than prior thin arcs that do not lead to onset, and that streamers that do not lead to onset form or intensify such quiet arcs. The newly formed or intensified quiet arc remains bright for a few to tens of minutes (∼20 min on average) until a subsequent streamer leads to substorm auroral onset along the pre‐existing arc. The pre‐onset sequence proposed here suggests that both types of streamers, which do and do not lead to substorms, enhance auroral luminosity near the equatorward boundary of the oval, and that a sufficiently intense, quiet time thin arc near the poleward edge of proton precipitation, likely corresponding to a large plasma pressure gradient in the near‐Earth plasma sheet, reflects important pre‐conditions for a precursor flow burst to trigger substorm auroral onset