Midlatitude Radar Observations of the July 2004 Geomagnetic Storm

1. Midlatitude Radar Observations of the July 2004 Geomagnetic Storm Melissa Meyer, Andrew Morabito, Zac Berkowitz, John Sahr University of Washington Electrical Engineering

2. the Manastash Ridge Radar Cascade Mountains E-region Plasma Density Structures 400-1100 km 400-1100 km FM Radio Transmitter Remote Receivers Reference Receiver 30 km 150 km

3. Radar Field of View

4. MRR Data Products Ground Clutter and Airplanes Power Scale: dB, uncalibrated Density Irregularity

5. Electric Field Structure via Coherent Radar • Coherent scatter from density irregularities caused by Farley-Buneman instability (threshold E required) • Treat irregularities as tracers for electric field structure • Millstone Hill Group reports linear relationship between coherent backscattered power & electric field strength (valid at ~440 MHz)

6. SAPS as Cause for MRR Backscatter • Due to its midlatitude location, MRR does not often observe auroral effects. • So what causes the irregularities? • We suspect “SAPS” (Sub-Auroral Polarization Stream): • M-I feedback instability, seeded by density gradients at the plasmapause (maps to midlatitude) • Poleward E; density trough (low conductivity); sunward drift • SAPS electric field can become very structured over short time periods (Foster et al., 2004)

7. July 2004 Magnetic Storm • MRR recorded semi-continuous data during 17-27 July 2004 • Two frequencies (96.5, 97.3 MHz) • Multiple antennas (interferometry)

8. VHF Coherent RadarBackscatter Intensity vs. Range and Time ~62o magnetic latitude 17 July 2004 (Kp 6) Mountains

9. SAPS Was There in July 2004: DMSP * DMSP High Latitude Space Weather Data courtesy of Fred Rich, AFRL, Hanscom AFB, Massachusetts

10. Auroral Precipitation Zone via DMSP SAPS Auroral Precip. Region ~61o

11. SAPS and the Auroral Region (Further East) Characteristic SAPS Density trough; E (ExB drift) enhancement Auroral Precip. Region ~60o

12. VHF Coherent RadarBackscatter Intensity vs. Range and Time Entire channelmotion: 140 m/s horizon cutoff “sub structure” motion: 415 m/s

13. 27 July 2004: Auroral Precip. / SAPS Channel Characteristic SAPS Density trough; E (ExB drift) enhancement ~59o

14. 27 July 2004: Backscatter Intensity vs. Range and Time Same quasi-periodic E field structure. (Kp 8) structure motion: ~850 m/s But faster, and no apparent “channel drift,” as before.

15. Measured SAPS Characteristics • Equatorward drift of entire channel: • Not always seen • Measured: 100 - 200 m/s • Drift of individual features: • 400 - 1000 m/s, equatorward • Large variability, seems to respond to disturbance level • Period of electric field enhancements: • Have seen 1 - 3 minutes; 10-20 minutes • (More observations needed.)

16. Similar Observations from other Radars • Millstone Hill • Channel movement ~150 m/s • Feature movement ~785 m/s • 3 - 5 min period • MHR resolution used: 10 km, 1 sec • Associated |E| oscillation with density oscillations (using GPS TEC measurements) *Foster, Erickson, Lind, and Rideout: GRL, 2004.

17. Fine Range Structure ~10 km periodic features (intensifications of |E|) Look like “SAID” events

18. Fine Range Structure • Interferometer: Echoes follow  aspect angle contour • Fine spatial structure persisted for ~3 hours on 17, 27 July during LT 17:00 - 20:00

19. Doppler Statistics from the July 2004 Storm • Gathered Doppler moment statistics from over 330,000 spectra • From 2 days during July 2004; disturbed conditions • Fitted each spectrum to Gaussian or Lorentzian curve via nonlinear least-squares (Levenburg-Marquardt)

20. Doppler Statistics from the July 2004 Storm:Mean Doppler vs. Spectral Width Notes • +/- Asymmetry • Faster + wider are correlated • Narrow, fast population

21. Doppler Statistics from the July 2004 Storm:Range vs. Doppler shift Notes • Speed-up at far ranges • Other structure visible (Lloyd’s Mirror? antenna pattern effects?)

22. Speed-up at Far Ranges:Individual Cases

23. Speed-up at Far Ranges (Why?) • Edge of auroral convection? • DMSP does show auroral precipitation dipping into MRR field of view, • But range speed-up is not discontinuous… • Observing Geometry? • Interferometric information not available (one antenna didn’t detect the faster echoes!)

24. Speed-up at Far Ranges (Why?) • At far ranges, shadow of Earth overtakes lower altitudes: only higher altitudes are visible. • At high E-region altitudes, temperature (cs) is greater and ions are more mobile. • Electron-ion drift (and E) must be greater to drive instability.

25. Other Features in Our Data… • Narrow, fast population: Examples • Often see spectra with 2nd, faster peak • Associated with fine range structure.

26. Other Features in Our Data… A shear in velocity / electric field over range

27. Summary • MRR often detects SAPS electric field structure(coherent radars at midlatitude are a good tool for learning about SAPS) • SAPS fields can develop very fine spatial structure (how?) • Faster spectra tend to be wider(& vice versa) • Faster echoes occur at higher altitudes. (Larger Vd required) • Passive radar is a versatile, useful tool.