Observations Of Temperature Gradient Observations Of Temperature Gradient Instabilities In The Plasmapause Region Instabilities In The Plasmapause Region Using The SuperDARN HF Wallops Radar And Using The SuperDARN HF Wallops Radar And Millstone Hill Millstone Hill CEDAR 2007 CEDAR 2007 P. J. Erickson, F. D. Lind P. J. Erickson, F. D. Lind Atmospheric Sciences Group Atmospheric Sciences Group MIT Haystack Observatory MIT Haystack Observatory R. A. Greenwald, K. Oksavik R. A. Greenwald, K. Oksavik Johns Hopkins University / APL Johns Hopkins University / APL
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Observations Of Temperature Gradient Instabilities In Observations Of Temperature Gradient Instabilities In The Plasmapause Region Using The SuperDARN HF The Plasmapause Region Using The SuperDARN HF
Wallops Radar And Millstone Hill Wallops Radar And Millstone Hill
CEDAR 2007 CEDAR 2007
P. J. Erickson, F. D. LindP. J. Erickson, F. D. Lind Atmospheric Sciences GroupAtmospheric Sciences Group MIT Haystack ObservatoryMIT Haystack Observatory
R. A. Greenwald, K. OksavikR. A. Greenwald, K. OksavikJohns Hopkins University / APLJohns Hopkins University / APL
Example of an Instability ProcessExample of an Instability ProcessF-region gradient-drift instability
High Density
Low Density
High Density
Low Density
B0
E0
V0= (E0xB0)/ B02
++
++
+
+ --
--
--
--
- E1
E1
E1
++
+
++
++
+
++
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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?
Examples of Ionospheric Scatter Examples of Ionospheric Scatter From Plasmasphere Boundary From Plasmasphere Boundary
LayerLayer
7 8-6+ 6+
1 4 3 4
1 1 1 2
1 2 1 1Jan 21, 2006
Beam 4
Jan 22, 2006Beam 4
Jan 23, 2006Beam 4
Sept 11, 2005 Beam 1
Potential Mechanism: Temperature Gradient Instability
• 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
Hudson and Kelley (1976)
TeNe
Millstone/Wallops Experiment to Identify Millstone/Wallops Experiment to Identify Source of Subauroral IrregularitiesSource of Subauroral Irregularities
MHO: 34 az, (18/28/48 el) + zenith focused on 55-60 inv @ 300 km
Wallops: 16 beam Doppler velocity scan. Millstone Hill is along beam indicated by the arrow.
TeZenith54 inv
Te48 el55.5 inv
Te28 el57.0 inv
Te18 el58.2 inv
Log(Ne)Zenith54 inv
Log(Ne)48 el55.5 inv
Log(Ne)28 el57.0 inv
Log(Ne)18 el58.2 inv
Millstone Hill Plasma Parameters In F Region: 2300 – 0430 UTC
2200 – 2340: Ground refracted scatter2340 – 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)
Temperature Gradient Cause: Trapped Electrons + Postsunset Cooling
(NOAA-17)
Gradient Drift Is Not Primary Mechanism: Millstone Plasma Velocities Agree with Wallops Irregularity Doppler Speeds
(* = Millstone Hill, red line = Wallops)
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
