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Observations of Temperature Gradient Instabilities in the Plasmapause Region

Study using SuperDARN HF Wallops Radar and Millstone Hill CEDAR data to analyze F-region gradient-drift instability phenomena and ionospheric scatter, with focus on temperature gradients and irregularities near the plasmapause.

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Observations of Temperature Gradient Instabilities in the Plasmapause Region

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  1. Observations Of Temperature Gradient Instabilities In The Plasmapause Region Using The SuperDARN HF Wallops Radar And Millstone Hill CEDAR 2007 P. J. Erickson, F. D. Lind Atmospheric Sciences Group MIT Haystack Observatory R. A. Greenwald, K. Oksavik Johns Hopkins University / APL

  2. - - + + - - + + - - + + - - + + - - + + - - - + + - - - + + - - - + + Example of an Instability Process F-region gradient-drift instability High Density Low Density High Density V0= (E0xB0)/ B02 E1 E0  E1 E1 B0 Low Density

  3. The Phenomena: Persistent Low-Velocity Wallops Echoes Very frequent (e.g. Feb 2006: 19 out of 27 observation days) Long duration (7+ hours per night) Low Doppler shift (30-90 m/s) Very small spectral width Low activity (Kp 0-2) Sub-auroral region (54-60 inv lat) Cause?

  4. 1 2 1 1 Jan 21, 2006 Beam 4 Jan 22, 2006 Beam 4 Jan 23, 2006 Beam 4 Sept 11, 2005 Beam 1 1 1 1 2 4 3 1 4 6+ 7 8- 6+ Examples of Ionospheric Scatter From Plasmasphere Boundary Layer

  5. Potential Mechanism: Temperature Gradient Instability Hudson and Kelley (1976) Te Ne • Growth in regions of strong density and temperature gradients • Temperature gradient drives drift into density gradient • Seen in the vicinity of SAR arcs at L=3.2 • Originally observed with OGO-6, OV1-17 electric field detectors at > 400 km • May seed other instabilities (Gradient drift) responsible for HF backscatter

  6. Wallops: 16 beam Doppler velocity scan. Millstone Hill is along beam indicated by the arrow. MHO: 34 az, (18/28/48 el) + zenith focused on 55-60 inv @ 300 km Millstone/Wallops Experiment to Identify Source of Subauroral Irregularities

  7. Millstone Hill Plasma Parameters In F Region: 2300 – 0430 UTC Log(Ne) Zenith 54 inv Log(Ne) 48 el 55.5 inv Log(Ne) 28 el 57.0 inv Log(Ne) 18 el 58.2 inv Te Zenith 54 inv Te 48 el 55.5 inv Te 28 el 57.0 inv Te 18 el 58.2 inv

  8. SuperDARN Wallops HF Backscatter + MHO Gradients 2200 – 0500 UTC 2006-02-22 2200 – 2340: Ground refracted scatter 2340 – 0140: GDI or trough wall or zonal gradient (seen before). TGI not active yet. 0140 onwards: TGI conditions present as Te gradient changes sign. Scatter weakens at higher beams as density decreases. TX frequency adjusted at 0410 UT – enhances scatter (refraction change)

  9. Temperature Gradient Cause: Trapped Electrons + Postsunset Cooling (NOAA-17)

  10. Gradient Drift Is Not Primary Mechanism: Millstone Plasma Velocities Agree with Wallops Irregularity Doppler Speeds (* = Millstone Hill, red line = Wallops)

  11. Summary • Low Doppler irregularities frequently observed near plasmapause by SD Wallops • Joint ISR – SD experiment identifies temperature gradient instability as primary driver (although GDI may contribute through cascade) • Postsunset low latitude cooling and high latitude trapped electron heating contribute to formation of temperature gradient • Possible Wallops monitor of plasmapause boundary?

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