Paper#: 29
Poster #:
Session Name: Plenary
Room: West Ballroom
Day: Thursday
Time: 8:55-9:20 a.m.
Abstract Title: Observed Timings Relevant to the Cause of Substorm Onset
PresentSurname: Lyons, L.
All Authors: L. Lyons
Abstract : There is general acceptance that auroral breakup and current wedge formation initiate at substorm onset on field lines within the near-Earth plasma sheet. After many years of debate, several studies completed prior to ICS-5 demonstrated that onset in this region results from processes initiating near breakup field lines and not the more distant plasma sheet. This conclusion has not been changed, leaving the question of what within the inner plasma sheet leads to onset. New observational results on this topic will be discussed. These include results obtained from CANOPUS photometer and imager observations of pre-onset and expansion-phase auroral arcs. Typically, one or more arcs are seen throughout the growth phase prior to onset. The observations show that breakup at onset does not occur along one of these arcs but instead generally occurs along a thin arc that forms a few minutes prior to onset equatorward of the growth phase arcs. The intensity of this breakup arc increases in intensity prior to the time normally identified as onset, and then increases dramatically. Arcs poleward of the breakup arc appear unaffected by onset until expansion-phase auroral activity moves poleward to the location of such arcs. Arcs poleward of the poleward-most extent of pseudo-breakup auroral activity show no affects of a pseudo-breakup. Also SuperDARN measurements of dayside convection will be presented for isolated onsets. These show that a reduction in large-scale convection is generally associated with onset. The formation of the breakup arc a few minutes prior to onset and the reduction of large-scale convection are fundamental aspects of the onset process that must be understood in order to understand substorm onset.