Statistical characterization of the large‐scale structure of the subauroral polarization stream

<jats:title>Abstract</jats:title><jats:p>The subauroral polarization streams (SAPS) are latitudinally narrow regions of westward directed flows observed equatorward of the evening sector auroral oval. Previous studies have shown that SAPS generally occur during geomagnetically disturbed conditions and exhibit a strong dependence on geomagnetic activity. In this paper, we present the first comprehensive statistical study of SAPS using measurements from the U.S. midlatitude Super Dual Auroral Radar Network (SuperDARN) radars. The study period spans January 2011 to December 2014, and the results show that SuperDARN radars observe SAPS over a broad range of activity levels spanning storm time and nonstorm conditions. During relatively quiet conditions (−10 nT <<jats:italic>D</jats:italic><jats:italic>s</jats:italic><jats:italic>t</jats:italic>< 10 nT) SAPS occur 15% of the time and tend to be localized to the midnight sector and centered above 60° magnetic latitude. As the activity level increases, the peak SAPS location shifts equatorward and duskward. During moderately disturbed conditions (−75 nT <<jats:italic>D</jats:italic><jats:italic>s</jats:italic><jats:italic>t</jats:italic> <− 50 nT) SAPS occur 87% of the time and tend to be centered at 20 magnetic local time (MLT) and below 60° magnetic latitude. This behavior has been encoded into a new empirical model which uses <jats:italic>D</jats:italic><jats:italic>s</jats:italic><jats:italic>t</jats:italic> as input to estimate the probability of SAPS occurrence at a given magnetic latitude and MLT. Similar to some previous studies, the variation of SAPS speed with MLT is found to be nearly linear at low to moderate levels of geomagnetic activity but becomes increasingly nonlinear near dusk sector as geomagnetic activity increases. We interpret this behavior as indicative of active ionosphere‐thermosphere feedback playing an important role in modulating SAPS speeds.</jats:p>