Composite Imaging of Auroral Forms and Convective Flows During a Substorm Cycle

Abstract

Measurements obtained with the electronically steerable Poker Flat Incoherent Scatter Radar (PFISR) and collocated all-sky camera were used to construct composite images of ionospheric convective flows and auroral forms during a substorm cycle (onset 26 March 2008, 1146 UT). PFISR was configured to sample an array of 5 × 5 regularly spaced beams on a pulse-by-pulse basis, from which velocity vectors were computed via statistical inversion of groups of beams. Flow fields were resolved at 30 km spatial resolution at 2 min temporal resolution over a 100 × 100 km field and then geographically registered with all-sky imagery recorded at 20 s cadence. An analysis of the composite images has revealed interesting contrasts between growth-, expansion-, and recovery-phase auroras, for example, (1) anticorrelation of ion velocity (electric field) and luminosity (plasma density, hence, conductance) in both space and time during growth phase and expansion phase; (2) identical flow (magnitude and direction) inside and outside the aurora during recovery phase; (3) a large tangential flow component along auroral boundaries during both growth and recovery phase (consistent with electric field directed into the aurora), irrespective of the orientation of the arc boundary; and (4) large relative drift (∼2 km/s) between auroral forms and convective flow during recovery phase. These features are interpreted in the context of previous ground-based and space-borne observations. Future PFISR experiments are expected to enable flow field construction at 30 s cadence, which will resolve Alfvén transit time dynamics to putative substorm initiation regions and significantly clarify the observations presented herein.