1. SuperDARN Observations of ULF Pulsations During a Substorm Expansion Phase Onset N. A. Frissell, J. B. H. Baker, J. M. Ruohoniemi, L. B. N. Clausen, R. A. Greenwald Department of Electrical and Computer Engineering, Virginia Tech With acknowledgements to I. J. Rae, Z. C. Kale, L. Kepko, M. Lester, A. Grocott, T. K. Yeoman, S. E. Milan, M. L. West, the THEMIS Team, and our international SuperDARN partners. Group Meeting – 27 September 2010

2. Pi2 Motivation What are Pi2’s? – ULF Geomagnetic Pulsation – Irregular Waveform – Period of T = 40 – 150 s [Jacobs and Sinno, 1960] Why are they important? – Pi2 are a characteristic signature of substorms. – Understanding of geomagnetic/geoelectric waves leads to a better understanding of space current systems, energy transport, etc.

3. Pi2 Motivation (2) Pi2s have been studied for many years Many models/mechanisms of Pi2 generation exist. Some include: – Transient Response Model [Baumjohann and Glassmeier, 1984] – Field Line Resonance Model [Takahashi et al., 1988] – Global Cavity Mode [Kivelson and Southwood, 1986] – Plasmapause surface modes [Chen and Hasegawa, 1974]

4. Pi2 Motivation (3) BUT… – Not all mechanisms are fully understood. – Multiple mechanisms can occurs at once. – Most mechanisms are regional. AND… – In the past, almost all ground-based observations were made with magnetometers.

5. SuperDARN THEMIS Mode SuperDARN THEMIS Mode! Operates during THEMIS tail conjunctions Dwell time per beam halved to 4 seconds. The radar returns to a camping-beam between each successive beam of the normal wide area scan. This provides: 8 second time resolution on one camping beam. 2 minute hemispheric spatial coverage Beam 8 (No Pulsations) Beam 7 – Camping Beam (Pulsations Observed) 22 FEB 2008

6. Event Conditions (1) AL AE AU [nT]

7. POES auroral precipitation flux at 0430-0530 UT (blue colors) Locations of ground magnetometers (brown asterisks) T96 Footprints of two THEMIS spacecraft (letters D and E) Estimates of the auroral oval (grey circles) [Holzworth and Meng, 1975] WW Traveling Surge and SCW FACs (pink region with encircled dots) [Liu et al., 2009] Plasmapause (black dotted box) Event Conditions (2)

8. Radar and Magnetometer Pi2s Blackstone RTI Plot L=3.16 2240 MLT (55.79˚N, 12.49˚W) AACGM` Remus, MI (317˚ Comp) L=2.99 2240 MLT (54.65˚N, 12.63˚W) AACGM [˚ AACGM Latitude] [m s -1 ] [nT] BKS Bm7, RG 21 L=3.16 2240 MLT (55.79˚N, 12.49˚W) AACGM` Remus, MI (HDZ) L=2.99 2240 MLT (54.65˚N, 12.63˚W) AACGM

9. THEMIS Satellite Locations From Liu et al. [2009]

10. THEMIS-Observed BBFs Vx Vy Vz THEMIS D L= 6.24 2247 MLT (66.40˚N, 10.89˚E) AACGM THEMIS E L= 6.77 2227MLT (67.39˚N, 17.26˚E) AACGM

11. StationT1T2T3T4Avg BKS138 s192 s165 s RMUS138 s164 s157 s177 s159 s THM-D135 s Comparing the Three Data Sets The first inter-pulse period (T1) matches up really, really well!! 0440:54 0437:14 0444:02 0438:32 0438:36 0440:50 0439:29 0443:58 0446:35 0449:32 1. BKS Radar 7,21 L=3.16 2240 MLT (55.79˚N, 12.49˚W) AACGM 2. RMUS (317˚ Comp) L=2.99 2240 MLT (54.65˚N, 12.63˚W) AACGM 3. THM-D (Ion Vel X-GSM) L= 6.24 2247 MLT (66.40˚N, 10.89˚E) AACGM [m s -1 ] [km s -1 ] [nT] -100 100 4 -6 -400 600 0 0 0

12. Plasmapause Location Zoe Kale performed a cross-phase analysis of this data that estimates the observed pulsations are located in the plasmapause between L- shell 3.0-3.71 (54-58 Λ ).

13. Plasmaspheric Compression in Radar Measurements

14. BBF Braking and Pi2 Generation From Kepko et al. [2001]

15. Estimate of Propagation Time 22 Feb 2008 BBF Starts at THM-D at 0437:14 UT X = -10.9 Re GSM Vx = 570 km/s BBF Travel Time: 10.2 s BBF Starts at THM-D at 0437:14 UT X = -10.9 Re GSM Vx = 570 km/s BBF Travel Time: 10.2 s Travels as fast mode wave starting at BBF Braking Point at X = -10 Re GSM with local Alfvén Velocity. Alfvén Travel Time: 69.5 s Travels as fast mode wave starting at BBF Braking Point at X = -10 Re GSM with local Alfvén Velocity. Alfvén Travel Time: 69.5 s Compressional Wave arrives at L-Shell of highest BKS measurement at 0438:33 UT. Total travel time: 79.7 s Compressional Wave arrives at L-Shell of highest BKS measurement at 0438:33 UT. Total travel time: 79.7 s T96 Parameters: DST = 2.5 nT pDyn = 1.4 nPa IMF By GSM = 3.5 nT IMF Bz GSM = -0.5 nT Particle mass density: Based on R-3 dependence from center of Earth [Chi et al., 2001] and density measured at THM-D (n = 0.5 cm-3) and approx. mass (m = 2E-27 kg).

16. Conclusions Data from the Blackstone SuperDARN radar, THEMIS magnetometer network, and THEMIS Satellites suggest that the Pi2 observed near Remus, MI during a substorm on 22 Feb 2008 at 0436 UT were caused by a compression of the plasmapause due to a earthward moving BBF braking against the inner magnetosphere and sending compressional energy toward earth. SuperDARN type radars are now effective tools for the study of mid-latitude ULF waves.