Possible connection of polar cap flows to pre- and post-substorm onset PBIs and streamers

Abstract

[1] Recent analysis of a short period of observations has led to the hypothesis that enhanced meso-scale flows from well within the region of open polar cap field lines may cross the nightside polar cap boundary into the closed field line region and contribute to the triggering of equatorward (earthward) meso-scale flows across the ionospheric (equatorial) portion of plasma sheet fields lines and lead to PBIs and streamers. This includes the streamers that have been postulated to bring new plasma equatorward (earthward) and lead to substorm onset. Meso-scale structure of flow within the polar cap, often studied near the dayside polar cap boundary, has not previously been generally recognized as significant within the nightside polar cap. Here we have taken advantage of new capabilities to measure polar cap convection by the Resolute Bay incoherent scatter radar and the Rankin Inlet PolarDARN radar, coordinated with THEMIS all-sky imager observations, to study flow measurements from well within the polar cap to near the polar cap boundary. We present evidence that flow structures moving from the polar cap toward the nightside polar cap boundary may be important for triggering the flows that lead to substorm onset streamers. The new observations also have given evidence that the flow structures come from deep within the polar cap, and have given unexpected evidence that a continuation of flow structures moving from the polar cap toward the nightside polar cap boundary after substorm onset may be important in controlling the poleward expansion and duration of post-onset auroral activity.

Statistical relationships between enhanced polar cap flows and PBIs

“…For our database, we require favorable sky condition and relatively good radar coverage, especially between the 0 and 11 range gates we use in this study (covering the magnetic latitudinal range ~74°–78.5° MLAT), which are located just poleward of the most probable latitudes of the auroral poleward boundary [ Nishimura et al ., ]. The short range of these echoes indicates that the radar signals only travel a short distance before getting backscattered [see, e.g., Koustov et al ., ], so that the altitude of backscatter is expected to be in the E region ionosphere [ Gorin et al ., ; Lyons et al ., , Figure 11], and thus, the flow magnitude will be less than the full E × B drift speed and limited below the ion acoustic speed (~400 m/s) [ Haldoupis , ; Koustov et al ., ] and the mean flow angle will be shifted from the E × B direction by ~20° [ Gorin et al ., ]. However, these do not influence on our results because we only study relative changes in magnitudes of flows directed toward the auroral poleward boundary.…”

“…[] and Lyons et al . [], and the THEMIS mode data newly used here allowed us to identify the time lag with a much higher accuracy.…”

“…[] further investigated the timing using a limited number of events and showed that PBIs can be preceded by longitudinally narrow (~0.2 h magnetic local time (MLT)) and transient flow channels within the polar cap region that are incident upon the nightside poleward boundary. Such flows may have propagated from high latitude deep within the polar cap [ Lyons et al ., ]. These case studies suggest an unexpected but promising connection between enhanced polar cap flows and PBIs.…”

“…[] and Lyons et al . [], the equatorward directed polar cap flow can be further followed by an ionospheric fast flow within the auroral oval as well as dipolarizations in the plasma sheet. These satellite measurements imply an association between PBIs and fast flows that propagate across the separatrix and bring new plasma into the plasma sheet, consistent with ionospheric observations.…”

Analysis of close conjunctions between dayside polar cap airglow patches and flow channels by all-sky imager and DMSP

“…Although those studies did not investigate whether the patches in the nightside polar cap are associated with mesoscale flows, recent radar observations have shown the existence of mesoscale fast flows deep within the nightside polar cap. Those fast flows propagate toward the nightside auroral oval and influence nightside auroral intensifications (Nishimura et al 2010;Lyons et al 2011;Zou et al 2014). The similarities between mesoscale flow channels in the dayside cusp and within the nightside polar cap, as well as the patch propagation from the dayside to nightside polar cap, suggest that mesoscale fast flows in the dayside oval propagate deep into the polar cap (Wilder et al 2012 and references therein).…”