Department of Electrical and Computer Engineering, Virginia Tech SuperDARN Observations of ULF Pulsations During the Expansion Phase Onset of THEMIS Substorms J.B.H. Baker, N.A. Frissell, J.M. Ruohoniemi, R.A. Greenwald Department of Electrical and Computer Engineering, Virginia Tech With acknowledgements to Larry Kepko, Jonny Rae, Zoe Kale, the THEMIS Team, and our international SuperDARN partners.

SuperDARN THEMIS Mode THEMIS mode camping beams (Blue) During THEMIS tail conjunctions SuperDARN radars are running a special THEMIS mode to increase temporal resolution during substorm onsets: 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. The THEMIS-mode simultaneously provides: Hemispheric spatial coverage (2-min). Higher temporal resolution on one camped beam per radar (8 seconds). THEMIS mode camping beams (Blue)

Event 1: February 22nd 2008 Small enhancement in AL index at approximately 0440-0450 UT is the first sign of geomagnetic activity on this particular day. At ~0437 UT THEMIS spacecraft measured two bursts of earthward convection.

Event 1: Ground Magnetometers Cross-phase calculations of the magnetometer data estimate the Plasmapause is located at L-shell ~ 3.0-3.71 (54-58Λ) (Courtesy of Zoe Kale)

Event 1: Ground Magnetometers and THEMIS spacecraft measurements Bx SWNO Ground Mag By Bz THEMIS-D ESA (-10.9,2.3,-3.4) Re Vx Vy THEMIS-E ESA (-10.1,3.1,-3.5) Re Vz Bx THEMIS-D FGM By Bz THEMIS-E FGM 0400UT 20080222 0600 UT

Event 1: Blackstone Measurements Beam-8: normal scan data (2-minutes) Beam-7: camping beam data (7-second) 0430 UT 0440 UT 0450 UT At ~0437 UT Blackstone measures oscillations on beam-7. Neighbouring beam-8 sees no evidence of oscillations. These measurements are near the Plasmapause.

Event 1: Goose Bay Measurements 0430 UT 0440 UT 0450 UT At ~0441 UT Goose Bay sees an onset of pulsations followed by a general increase in the strength of poleward convection. Unlike Blackstone, the pulsations are moving equatorward. The magnitudes are larger than those seen at Blackstone. These measurements are poleward of the plasmapause.

Event 1: Shear Alfven Waves? Bx Pinawa Magnetometer By Bz Blackstone Beam-7 0430 UT 0450 UT The similarities between the oscillations seen by the Blackstone radar and the ground magnetometer at Pinawa suggest that the oscillations may be associated with shear Alfven waves. Future work will examine this possibility in more quantitative terms.

Summary: Event 1 We have investigated electric pulsations during the onset of a substorm observed by THEMIS spacecraft on February 22nd, 2008. Two flow bursts were measured by the THEMIS-D and THEMIS-E spacecraft starting at approximately 0436 UT. The plasmapause location from cross-phase technique is L=3.0-3.7 Blackstone started measuring pulsations in the vicinity of the plasmapause at approximately 0438 UT with similar characteristics to magnetic pulsations measured by the Pinawa magnetometer. Goose Bay started measuring pulsations at 0441 UT in a region poleward of the plasmapause. The pulsations moved equatorward. The pulsations measured by both radars were only observed on the THEMIS mode camping beams.

Event 2: January 29th 2008 Isolated onset at ~0714 UT followed by second onset at ~0742 UT and then multiple intensifications. THEMIS-D measurements show 3 dipolarizations at times corresponding to enhancements in the AL index.

Event 2: 1st Onset at 0714 UT Prince George Beam-12

Event 2: 2nd Onset at 0742 UT Prince George Beam-12 Kodiak Beam-7 Saskatoon Beam-6

Summary: Event 2 We have investigated electric pulsations during two substorm onsets observed by the THEMIS spacecraft on January 29th, 2008. The first onset was an isolated substorm that occurred at approximately 0714 UT. Short-lived pulsations measured by the Prince George radar on its camping beam have a clear correspondence with similar oscillations measured by ground magnetometers. The second onset occurred at approximately 0742 UT. Pulsations were measured on the camping beams of the Prince George, Kodiak and Saskatoon radars but the relationship to ground magnetometer measurements is less clear because of the increased activity. The electric pulsations were only observed on the 8-second beams.

Summary During THEMIS tail conjunctions each SuperDARN radar is running a special THEMIS camping beam mode which provides: Hemispheric context via 16-beam scans. High resolution 8-second data on a single camping beam. Pulsations have been identified on the camping beams during the expansion phase onset of two THEMIS substorms. The electric pulsations have similar characteristics to magnetic pulsations measured by ground-based magnetometers. The pulsations were not observed on the neighboring 2-minute beams. Future work will investigate the relative magnitude and phase of the oscillations in the radar and magnetometer data in an effort to better understand the nature of the waves. Any helpful suggestions are most welcome!