Relation of substorm breakup arc to other growth‐phase auroral arcs

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Abstract

High‐resolution CANOPUS meridian‐scanning photometer and all‐sky imager observations of preonset and expansion‐phase auroral arcs are analyzed for expansion‐phase onsets that evolve into full substorms and into pseudobreakups. One or more arcs are seen across the sky throughout the growth phase prior to onset. The onsets we have examined indicate that auroral breakup at onset does not generally occur along one of these arcs but instead often occurs along a thin breakup arc that forms equatorward of all growth phase arcs a few minutes prior to onset. The intensity of this breakup arc increases monotonically for the few minutes prior to the time normally identified as substorm onset and then typically increases dramatically. These results imply that the processes responsible for auroral breakup initiate a few minutes prior to the time normally identified as substorm expansion‐phase onset. We also find that arcs poleward of the arc that breaks up appear to be unaffected by substorm onset until expansion‐phase auroral activity moves poleward to the location of such arcs. Arcs poleward of the poleward‐most extent of pseudobreakup auroral activity show no apparent effects of a pseudobreakup. These results imply that the process that initiates the onset of substorms does not require the occurrence of plasma sheet changes, significant enough to affect magnetosphere‐ionosphere electrodynamics, along field lines that cross the equator tailward of the substorm onset region.

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