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.