1. Large-scale observations of the spatial and temporal dynamics of Subauroral Polarization Streams
N.Pramodkumar (1), J.B.H.Baker (1), J.M.Ruohoniemi (1), L.B.N.Clausen (2), B.S.R.Kunduri (1), E.G.Thomas (1)
(1) Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University
(2) University of Oslo, Department of Physics

2. Intent of the study
The Sub-Auroral Polarization Stream (SAPS) is a narrow, intense and persistent westward (sunward) ionospheric convection flow channel observed equatorward of the auroral electron precipitation boundary, predominantly on the nightside. (Foster and Burke, 2002)
SAPS intensification during the recovery phase of a substorm, preceded by a weak (often non-existent) response to the substorm onset is a well-documented phenomenon, observed during periods of low Kp. (Anderson et.al., 1993, Makarevich et.al., 2011)
The recent expansion of the SuperDARN radars to mid-latitudes facilitates unprecedented large-scale observations of SAPS over 10 hours of MLT with high temporal and spatial resolution.
The L-shell fitting technique implemented across neighboring pairs of mid-latitude radars can be used to analyze the dynamics in the strength and direction of the two-dimensional SAPS flow velocities at separate magnetic longitudes. (Villian et al., 1987, Ruohoniemi et al., 1989, Clausen et al., 2012)
SAPS can be identified as elevated flows lying equatorward of the auroral electron precipitation boundary specified by the NOAA POES satellites
In this study we analyze the life cycle of a quiet-time SAPS event between 4 UT and 8 UT on 14th May 2012, observed by six adjacent mid-latitude SuperDARN radars, using the L-shell fitting technique. We compare our findings with the magnetic field perturbation measurements obtained from the CARISMA magnetometers.

3. The SuperDARN radars used in this study
Radar Name
Geomagnetic
Start Year
Code
Scan Direction
Lat
Lon
Boresite
Christmas Valley East, OR
49.5
-58.3
40.2
2011
CVE E
Christmas Valley West, OR
-31.2
CVW W
Fort Hays East, KS
48.9
-32.2
41.3
2009
FHE
Fort Hays West, KS
-32.3
FHW
Wallops Island, VA
48.7
0.8
46.7
2005
WAL
Blackstone, VA
48.2
-2.7
-41.5
2008
BKS
The Christmas Valley radars (CVE & CVW), the Fort Hays radars (FHE & FHW), and the Blackstone (BKS) and Wallops Island (WAL) radars together form an unbroken chain covering over 10 hours of local time.
The location and extent of these radars facilitate large-scale observations of the spatial and temporal dynamics of Subauroral Polarization Streams.

4. Event Study May 14th, 2012 (4:00 UT – 8:00 UT)
Quiet day as identified by the GFZ German Research Centre for Geosciences, based on the Kp index.
SAPS observed between 4 and 8 UT by mid-latitude SuperDARN radars.
IMF Bx and Sym-H remains at a steady -5 nT and -30 nT respectively, and Kp reduces from 3 to 1 in this interval.
The SuperDARN observations of the evolution of this SAPS event coincides with substorm activity recorded by the AU, AL indices and the CARISMA magnetometers.

5. Event Study Continued….. May 14th, 2012, 06:55 UT
Christmas Valley East – All scatter plot (Fitex)

6. Event Study Continued….. May 14th, 2012, 06:55 UT
Christmas Valley West – All scatter plot (Fitex)

7. Event Study Continued….. May 14th, 2012, 06:55 UT
Christmas Valley West and Christmas Valley East
Ionospheric scatter plot

8. Event Study Continued….. May 14th, 2012, 06:55 UT
L-shell fitting in action : 62⁰ latitude and +/- 1.5 MLT from the average radar location (-58⁰).
Cosine fit of the line of sight velocities extracted from the ionospheric scatter observed by the Christmas Valley radars, as a function of radar beam azimuth.
Outliers (shown in grey) are removed through an iterative μ+1σ filtering technique

9. Event Study Continued….. May 14th, 2012, 06:55 UT
Stack plot of L-shell fitting results from 55⁰ to 70 ⁰ latitude

10. Event Study Continued….. May 14th, 2012, 06:55 UT
L-shell fitting results overlaid on ionospheric scatter (greyscale)
Peak velocity of the SAPS observed at 62⁰ latitude

11. Event Study Continued….. May 14th, 2012, 06:55 UT
NOAA POES Satellite passes overlaid on ionospheric scatter from all six mid-latitude radars
Curve fitting (red dashes) estimates the auroral electron precipitation boundary from the POES Electron Flux measurements.

12. Event Study Continued….. May 14th, 2012, 06:55 UT
L-shell fitting results and POES auroral electron precipitation boundary used to analyze the spatial and temporal dynamics of the SAPS channel as well as of the characteristic SAPS velocities.

14. Event Study Continued….. May 14th, 2012, 3:00 – 9:00 UT
Vector representation of peak velocities from the L-shell fitting results

15. Event Study Continued….. May 14th, 2012, 0 – 13:00 UT
Peak velocities from L-shell fitting (left-axis) and L-shell vector count (right-axis)

16. Event Study Continued….. May 14th, 2012, 3:00 – 9:00 UT
Peak velocities from L-shell fitting as a function of MLT and UT (color-coded in UT)

17. Event Study Continued….. May 14th, 2012, 0 – 13:00 UT
CARISMA magnetometer data – RANK (Rankin Inlet), FCHU (Fort Churchill) and ISLL (Island Lake).

18. Event Study Continued….. May 14th, 2012, 0 – 13:00 UT
CARISMA magnetometer data from RANK (Rankin Inlet) compared with total L-shell vector count computed using scatter from the six mid-latitude SuperDARN radars.

19. Summary and future directions
Mid-latitude SuperDARN radar observations were made for a quiet-day SAPS event on May 14th 2012:
L-shell fitting was performed on the line of sight velocities extracted from the ionospheric scatter observed by pairs of mid-latitude SuperDARN radars.
The equatorward auroral electron precipitation boundary specified by the NOAA POES satellites was used to verify the subauroral nature of the westward velocities observed by the mid-latitude SuperDARN radars in the nighttime ionosphere over Northern America.
The peak velocities extracted from the results of the L-shell fitting revealed a high level of correlation between the independent measurements of the three pairs of radars.
The count of L-shell vectors (westward) correlates well with the magnitude of the peak westward SAPS velocity extracted from the L-shell fitting results.
The spike in the count of L-shell vectors and peak velocity magnitudes falls well within the substorm activity period recorded by the RANK magnetometer (CARISMA), which provides further evidence to support our current understanding of substorm driven SAPS.
The L-shell fitting technique in conjunction with the POES equatorward auroral electron precipitation boundary can be used to accurately identify SAPS events and also to examine the spatial and temporal extent of individual SAPS events.
The wide local time coverage of the mid-latitude SuperDARN radars combined with the ability to dissect SAPS events using the L-shell fitting technique can be used to provide more insight into the life cycle of SAPS, its various geomagnetic drivers and various other aspects.

20. References
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, 2012.
Makarevich, Roman A., A. C. Kellerman, J. C. Devlin, H. Ye, L. R. Lyons, and Y. Nishimura. “SAPS Intensification During Substorm Recovery: A Multi-instrument Case Study.” J. Geophys. Res., 116, A11, 2011
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, 1993.
Foster, J. C., and W. J. Burke, SAPS: A new characterization for subauroral electric fields, Eos AGU Trans., 83(36), 393, 2002.
Ruohoniemi, J. M., R. A. Greenwald, K. B. Baker, J.-P. Villain, C. Hanuise, and J. Kelly. “Mapping High-latitude Plasma Convection with Coherent HF Radars.” J. Geophys. Res., 94, A10, 1989.
Villain, J. P., R. A. Greenwald, K. B. Baker, and J. M. Ruohoniemi. “HF Radar Observations of E Region Plasma Irregularities Produced by Oblique Electron Streaming.” J. Geophys. Res., 92, A11,