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

Abstract

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 <Dst< 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 <Dst <− 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 Dst 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.

An Auroral Boundary‐Oriented Model of Subauroral Polarization Streams (SAPS)

“…We have also presented a model of the equatorward edge of electron precipitation with which the subauroral potential model can be combined. A summary of statistical results is as follows: The variation of SAPS latitude with activity level and MLT agrees with previous statistical studies (e.g., Erickson et al, ; Foster & Vo, ; Kunduri et al, ; Wang et al, ). SAPS latitude decreases closer to midnight due to the decreasing latitude of the auroral boundary.…”

“…The total subauroral potential drop also maximizes at 1800–2000 MLT. Near solstice, the total subauroral potential drop and SAPS width is maximized between 1800 and 2000 MLT, but the maximum drifts are found between 1600 and 1800 MLT (see Figures and ). The latitudinal shape of the median SAPS drift profile is MLT dependent (see Figure ). SAPS drifts have a higher amplitude during equinoxes than during solstices (see Figures and ). There is an approximately linear decay in the maximum westward SAPS drifts in MLT from dusk to midnight in agreement with previous statistical studies (Erickson et al, ; Foster & Vo, ; Kunduri et al, ) and in disagreement with the SAPS event observed simultaneously by SuperDARN radars over 6 hr of MLT and 3 hr in universal time in Clausen et al (), which decayed exponentially (see Figure ). SAPS amplitude and width in the dusk sector vary inversely with F10.7 solar radio flux. (see Figure )…”

“…These drifts are identified as subauroral with an auroral boundary model fit to all energetic particle precipitation boundaries measured by the National Oceanic and Atmospheric Administration Polar Orbiting Environment Satellites (POES) satellites in ±40 min. The average models of Kunduri et al () may avoid the contribution of low occurrence frequency SAID, which tend to pull the average drifts significantly from the mode (see Figure ), and agree more with our model than FV02, due to the spatial and temporal averaging effects of the fitting techniques used to measure global ionospheric convection with SuperDARN radars (Ruohoniemi & Baker, ).…”

Bifurcated Region 2 Field‐Aligned Currents Associated With Substorms

“…This polarization electric field leads to a strong E x B drift, which forms the fast westward drifting SAPS flow (Kunduri et al, 2017Landry & Anderson, 2018;Lejosne et al, 2018). During the growth phase, the R1/R2 currents progress equatorward as the polar cap expands.…”

Statistical Study of Nightside Quiet Time Midlatitude Ionospheric Convection

“…The midlatitude expansion of SuperDARN radars over the past 10 years has provided new opportunities to study midlatitude ionospheric convection over large areas and with unprecedented spatial resolution and statistical significance. However, most studies of midlatitude convection using the midlatitude SuperDARN radars have focused on features that occur during geomagnetically disturbed conditions such as the high‐speed subauroral polarization streams (Clausen et al, ; Kunduri et al, , ; Oksavik et al, ). Only a handful of studies have focused on quiet time midlatitude convection (Baker et al, ; Zou & Nishitani, ).…”