Baker Tech SuperDARN Large-Scale Observations of the Sub-Auroral Polarization Stream (SAPS) From Mid-Latitude SuperDARN Radars Joseph B.H. Baker, Lasse B. N. Clausen, J. Michael Ruohoniemi Center for Space Science and Engineering Research Bradley Department of Electrical and Computer Engineering Virginia Tech

Joseph Baker Tech SuperDARN Motivation Sub-Auroral Polarization Streams (SAPS) are strong westward flows observed in the nightside ionosphere equatorward of the main auroral oval [e.g. Galperin et al., 1973; Yeh et al. 1991; Anderson et al., 1993; Foster and Burke, 2002]. The prevailing theories for SAPS associate them with enhanced plasma pressure in the inner magnetosphere but also ascribe an active feedback role to the ionosphere-thermosphere [e.g. Anderson et al., 1993]. Previous observations of SAPS have had low temporal and/or spatial coverage because they have used data from low-altitude satellites and incoherent scatter radars [e.g. Spiro et al., 1973; Erickson et al. 2011]. Recent expansion of the SuperDARN HF radar network to middle latitudes provides new opportunities to examine the large-scale dynamics of SAPS with unprecedented temporal resolution and magnetic local time coverage. SAPS events provide an excellent opportunity for testing the current generation of inner magnetosphere models because they provide a “large target”, which is easily parameterized, and should be accurately captured by the models.

Joseph Baker Tech SuperDARN The SuperDARN Radars The Super Dual Auroral Radar Network (SuperDARN) is an international network of high-frequency (HF) radars for researching the Earth’s upper atmosphere, ionosphere, and connection into geospace.

Joseph Baker Tech SuperDARN SAPS Event: April 9 th 2011 Moderate SW Vx Moderate IMF Southward IMF Bz Declining Kp index Substorm Activity Small Magnetic Storm Clausen et al., 2012

Joseph Baker Tech SuperDARN SAPS VLOS Time Series

Joseph Baker Tech SuperDARN SAPS VLOS Time Series

Joseph Baker Tech SuperDARN Auroral Oval and Trough Color-coded electron flux measurements from a POES spacecraft verify that the most intense flows measured by the SuperDARN radars are sub-auroral. Estimates of Total Electron Content (TEC) from GPS Receivers suggest that the high velocity flows are located inside the mid-latitude low conductance trough. Clausen et al., 2012

Joseph Baker Tech SuperDARN Longitudinal Dependence Vector time-series representation of peak SAPS flow direction and magnitude obtained by fitting VLOS vs Azimuth for each radar pair:  BKS-WAL (right)  FHE-FHW (center)  CVE-CVW (left) Direction of SAPS flows within each longitude sector is relatively stable over a time-scale of 3 hours. Moving westward across the 6-radar array the magnitude of the SAPS flows approximately doubles between each successive radar pair. Clausen et al., 2012

Joseph Baker Tech SuperDARN MLT-UT Variations Stacking together MLT profiles of the SAPS velocity and color-coding them by UT gives an overview of MLT-UT variations. It can be seen that the SAPS magnitude has a quasi-exponential dependence on MLT. Clausen et al., 2012

Joseph Baker Tech SuperDARN Model-Data Comparison An exponential dependence in the magnitude of SAPS velocities versus MLT is consistent with modeling. Figure shows logarithmic contours of inner magnetosphere plasma pressure from the Rice Convection Model (RCM) [Tofoletto et al., 2003] Colored dots show T96 equatorial mappings of maximum SAPS flow locations along a single beam per radar. Note the good correspondence in the location of the SAPS channel between model and measurements. Also note the uniform spacing of the pressure contours which is suggestive of exponential dependence. Clausen et al., 2012

Joseph Baker Tech SuperDARN 2012 SAPS Events DateSAPS UT IntervalStorm DstStorm Kp :00-12:00 UT -34 nT :30-07:00 UT -69 nT :00-10:00 UT -54 nT :00-04:30 UT -133 nT :00-03:30 UT -133 nT :00-08:00 UT -133 nT :30-05:30 UT -74 nT :00-05:00 UT -56 nT :00-07:00 UT -56 nT :00-07:00 UT -51 nT 4 NOTE: Many more events to analyze!!!!

Joseph Baker Tech SuperDARN Summary and Questions SuperDARN observations were examined for a SAPS event on April 9 th 2011: SAPS velocity direction was very stable over several hours of UT. SAPS velocity magnitude was variable but with good local time coherence. SAPS velocity magnitude had an exponential variation with MLT. With increased geomagnetic activity over the past year, mid-latitude SuperDARN radars are routinely observing SAPS with unprecedented coverage and resolution. The following questions can be asked about inner magnetosphere models: Do they accurately predict the latitude of the peak SAPS flows? Do they accurately predict the latitudinal width of the SAPS channel? Do they accurately predict the strength of the peak SAPS flows? Do they accurately predict the variability in the strength of the SAPS flows? Can models help us separate the magnetosphere and ionosphere influences? Answers to these questions will help us determine whether the physics coded in the models are accurate (e.g. magnetic and electric fields, ionospheric conductance)

Joseph Baker Tech SuperDARN References Anderson, P. C., W. B. Hanson, R. A. Heelis, J. D. Craven, D. N. Baker, and L. A. Frank (1993), A proposed production model of rapid subauroral ion drifts and their relationship to substorm evolution, J. Geophys. Res., 98, 6069, Clausen, L. B. N., J.B.H. Baker, J. M. Ruohoniemi, R.A. Greenwald, E.G. Thomas, S.G. Shepherd, E. Talaat, W.A. Bristow, Y. Zheng, A.J. Coster, S. Sazykin, “Large-scale observations of a subauroral polarization stream by mid-latitude SuperDARN radars: Instantaneous longitudinal velocity variations”, J. Geophys. Res., 117, A5, Erickson, P.J., F. Beroz, and M.Z. Miskin, Statistical characterization of the American sector subauroral polarization stream using incoherent scatter radar, J. Geophys. Res., 116, A00J21, Foster, J. C., and W. J. Burke, SAPS: A new characterization for subauroral electric fields, Eos AGU Trans., 83(36), 393, Galperin, Y., V. N. Ponomarev, and A. G. Zosimova (1974), Plasma convection in the polar ionosphere, Ann. Geophys., 30, 1, Spiro, R. W., R. A. Heelis, and W. B. Hanson, Rapid subauroral ion drifts observed by Atmosphere Explorer C, Geophys. Res. Lett., 6, 657, 1979 Tofoletto, F., S. Sazykin, R. Spiro, and R. Wolf, Inner magnetospheric modeling with the Rice Convection Model, Space Science Reviews, 107, 175, Yeh, H.-C., J. C. Foster, F. J. Rich, and W. Swider, Storm time electric field penetration observed at mid-latitude, J. Geophys. Res., 96, ,

Joseph Baker Tech SuperDARN