Influence of auroral streamers on rapid evolution of SAPS flows

Abstract:

An important manifestation of plasma transport in the ionosphere is Subauroral Polarization Streams or SAPS, which are strong westward flow lying just equatorward of the electron auroral oval and thus of enhanced ionospheric conductivities of the auroral oval. While SAPS are known to intensify due to substorm injections, recent studies showed that large variability of SAPS flow can occur well after substorm onset and even during non-substorm times. These SAPS enhancements have been suggested to occur in association with auroral streamers that propagate equatorward and then turn azimuthally, a suggestion that would indicate that plasma sheet fast flows propagate into the inner magnetosphere and increase subauroral flows. We present auroral images from the THEMIS ground-based all-sky-imager array and 2-d line-of-sight flow observations from the SuperDARN radars that share fields of view with the imagers to investigate systematically the association between SAPS and auroral streamers. We surveyed events from December 2007 to April 2010 for which high or mid-latitude SuperDARN radars were available to measure the SAPS flows, and identified 30 events. For streamers observed near the equatorward boundary of the auroral oval either while turning, or after having turned, westward, we find westward flow enhancements of ~200 m/s slightly equatorward of the streamers. A preliminary survey suggests that >90% of the streamers that turn westward lead to westward flow enhancements. We also characterize the SAPS flow channel width and timing relative to streamers reaching radar echo meridians. The strong influence of auroral streamers on rapid evolution of SAPS flows suggests that transient fast earthward plasma sheet flows can lead to westward SAPS flow enhancements in the subauroral region, and that such enhancements are far more common than just during substorms because of the frequent occurrences of streamers under various geomagnetic conditions.