Influence of auroral streamers on rapid evolution of SAPS flows
Monday, 23 May 2016
Bea Gallardo-Lacourt1, Yukitoshi Nishimura2, Larry R Lyons1, Vassilis Angelopoulos3, Eric Donovan4, J. Michael Ruohoniemi5 and Nozomu Nishitani6, (1)University of California Los Angeles, Los Angeles, CA, United States, (2)University of California Los Angeles, Department of Atmospheric and Ocean Science, Los Angeles, CA, United States, (3)University of California Los Angeles, Earth, Planetary, and Space Sciences, Los Angeles, CA, United States, (4)University of Calgary, Calgary, AB, Canada, (5)Virginia Tech, Blacksburg, VA, United States, (6)Nagoya University, Solar-Terrestrial Environment Laboratory, Nagoya, Japan
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. Previous studies suggested that the strongest SAPS occur when field-aligned currents (FAC) are more intense. 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, 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, 2-d line-of-sight flow observations from the SuperDARN radars, and FAC observations from AMPERE to investigate systematically the association between SAPS and auroral streamers. We surveyed events from December 2007 to May 2013 for which high or mid-latitude SuperDARN radars were available to measure the SAPS flows, and identified 104 events. For streamers observed near the equatorward boundary of the auroral oval, we find westward flow enhancements of ~700 m/s slightly equatorward of the streamers. Our statistical study shows that 60% of the westward flow enhancements are associated with streamers that reach close to the auroral equatorward boundary. In addition, we have found that the majority of the remaining events are flow increases associated with IMF changes. We have also characterized the SAPS flow channel width and timing relative to streamers reaching radar echo meridians. Using AMPERE, we also determine evolution of R2 FACs during SAPS flow enhancements associated with auroral streamers. 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 only during substorms because of the frequent occurrences of streamers under various geomagnetic conditions.
