1. Statistical Characterization of sub-auroral polarization stream using using large scale observations by mid- latitude SuperDARN radars B. S. R. Kunduri (1), J. B. H. Baker (1), J. M. Ruohoniemi (1), E. G. Thomas (1), L. B. N. Clausen (2), K. T. Sterne (1), P. J. Erickson (3) and J. C. Foster (3) (1) SuperDARN HF Radar Lab Virginia Tech (2) University of Oslo, Department of Physics (3) MIT Haystack Observatory

2. Traditional observations of SAPS SAPS are westward directed flows observed in the mid- latitude ionosphere equatorward of the auroral oval. Earlier observations of a SAPS channel were mostly through satellites (such as DMSP) and radars (Millstone Hill ISR). The observations were limited to a narrow longitudinal sector.

3. Large Scale observations from mid-latitude SuperDARN radars [Clausen et al., 2012] The recent expansion of SuperDARN network provides an extended longitudinal coverage in the mid-latitude ionosphere. Clausen et al., [2012] demonstrated that the mid-latitude chain can provide observations of SAPS spanning more than 6 hours in Magnetic Local Time.

4. SuperDARN SAPS observations between Jan-2011 and Aug-2012 Between Jan-2011 and Aug- 2012, about 160 SAPS events/days were observed in the midlatitude SD radars. SAPS were observed during varying geomagnetic conditions. Some events were observed when Dst index was greater than -10 nT and during some other events Dst index was lower than - 100 nT. However the probability of SAPS occurrence was observed to exhibit a exponential dependence on Dst. During lower Dst indices (less than -50 nT) SAPS were almost certainly observed.

5. Average occurrence characteristics SAPS events occurring during different Dst index ranges were grouped together and their characteristics were analyzed. Figure shows the variations in the probability of occurrence of a SAPS event with latitude and MLT. It is assumed during this characterization that a SAPS event occurs during a given Dst-index.

6. Variations in the most likely location of the SAPS channel with Dst index SAPS channel moves equatorwards and duskwards with decreasing Dst index. During disturbed geomagnetic conditions asymmetric ring current moves more duskwards and Earthwards [Tsyganenko., 2000]. SAPS channel was more polewards towards dusk than near midnight MLT.

7. Occurrence characteristics with respect to KP-index

8. Latitudinal variations with KP-index

9. Probability of occurrence can be represented with a 2d gaussian fit The bottom panel of the Figure presents the average characteristics of SAPS when Dst index is between -25 and -50 nT. The top panel shows a two dimensional gaussian fit to the data.

10. SAPS model

11. Comparison with actual observations Case-1 : Feb 14, 2013 (Dst ~ -30 nT)

12. Case-2 : Oct 9, 2012 ( Dst ~ -90 nT )

13. Characteristics of Velocities As suggested by Clausen et al., [2012] the velocities appear to exhibit a exponential drop with MLT. Variations in velocity with latitude suggest the presence of a SAID like feature when Dst index is very low (less than - 50 nT). The observations suggest there could be two different mechanisms responsible for SAPS 1) The traditional SAPS when Dst index < -50nT. 2) SAPS occurring during relatively quiet conditions.

14. Conclusions Mid-latitude SD radars can make observations of SAPS extending over 6 hours in MLT. Between Jan-2011 and Aug-2012 about 160 SAPS days/events were identified in the mid-latitude SuperDARN radars ( WAL, BKS, FHE, FHW, CVE, CVW ). These events occurred during widely varying geomagnetic conditions. Dst index values varied between -100 nT and 10 nT. SAPS was found to occur with a high probability during geomagnetically disturbed days ( Dst < -50 nT) and the probability fell exponentially as Dst Index increased (towards more positive values). The location of SAPS channel was observed to move further equatorward and duskward with decreasing Dst index. The variations in the location of a SAPS channel were modeled as a function of Dst index.