midlatitude radar observations of the july 2004 geomagnetic storm
DESCRIPTION
Midlatitude Radar Observations of the July 2004 Geomagnetic Storm. Melissa Meyer, Andrew Morabito, Zac Berkowitz, John Sahr University of Washington Electrical Engineering. the Manastash Ridge Radar. Cascade Mountains. E-region Plasma Density Structures. 400-1100 km. 400-1100 km. - PowerPoint PPT PresentationTRANSCRIPT
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Radar RemoteSensing Laboratory
University of Washington
Melissa Meyer, Andrew Morabito,Zac Berkowitz, John Sahr
University of Washington Electrical Engineering
Midlatitude Radar Observations of the July 2004 Geomagnetic Storm
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October 15, 2003 2Radar RemoteSensing Laboratory
University of Washington
RemoteReceivers
FM RadioTransmitter
ReferenceReceiver
30 km 150 km
E-region Plasma Density Structures
400-1100 km 400-1100 km
Cascade Mountains
theManastashRidgeRadar
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October 15, 2003 3Radar RemoteSensing Laboratory
University of Washington
Radar Field of View
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October 15, 2003 4Radar RemoteSensing Laboratory
University of Washington
MRR Data Products
Ground Clutter and Airplanes
Density Irregularity
Power Scale:dB, uncalibrated
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October 15, 2003 5Radar RemoteSensing Laboratory
University of Washington
• 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)
Electric Field Structure via Coherent Radar
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October 15, 2003 6Radar RemoteSensing Laboratory
University of Washington
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)
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October 15, 2003 7Radar RemoteSensing Laboratory
University of Washington
July 2004 Magnetic Storm
• MRR recorded semi-continuous data during 17-27 July 2004
• Two frequencies (96.5, 97.3 MHz)
• Multiple antennas (interferometry)
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October 15, 2003 8Radar RemoteSensing Laboratory
University of Washington
VHF Coherent RadarBackscatter Intensity vs. Range and Time
17 July 2004(Kp 6)
~62o
magneticlatitude
Mountains
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October 15, 2003 9Radar RemoteSensing Laboratory
University of Washington
SAPS Was There in July 2004: DMSP
* DMSP High Latitude Space Weather Data courtesy of Fred Rich, AFRL, Hanscom AFB, Massachusetts
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October 15, 2003 10Radar RemoteSensing Laboratory
University of Washington
Auroral Precipitation Zone via DMSP
Auroral Precip. Region ~61o
SAPS
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October 15, 2003 11Radar RemoteSensing Laboratory
University of Washington
SAPS and the Auroral Region (Further East)
Auroral Precip. Region ~60o
Densitytrough;
E (ExB drift) enhanceme
nt
Characteristic SAPS
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October 15, 2003 12Radar RemoteSensing Laboratory
University of Washington
VHF Coherent RadarBackscatter Intensity vs. Range and Time
horizoncutoff
Entire channelmotion: 140 m/s
“sub structure” motion: 415 m/s
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October 15, 2003 13Radar RemoteSensing Laboratory
University of Washington
27 July 2004: Auroral Precip. / SAPS Channel
~59o
Densitytrough;
E (ExB drift) enhanceme
nt
Characteristic SAPS
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October 15, 2003 14Radar RemoteSensing Laboratory
University of Washington
27 July 2004: Backscatter Intensity vs. Range and Time
(Kp 8)Same quasi-periodic E field structure.
But faster, and no apparent “channel drift,” as before.
structure motion: ~850 m/s
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October 15, 2003 15Radar RemoteSensing Laboratory
University of Washington
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.)
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October 15, 2003 16Radar RemoteSensing Laboratory
University of Washington
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.
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October 15, 2003 17Radar RemoteSensing Laboratory
University of Washington
Fine Range Structure
~10 km periodic features (intensifications of |E|)Look like “SAID” events
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October 15, 2003 18Radar RemoteSensing Laboratory
University of Washington
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
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October 15, 2003 19Radar RemoteSensing Laboratory
University of Washington
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)
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October 15, 2003 20Radar RemoteSensing Laboratory
University of Washington
Doppler Statistics from the July 2004 Storm:Mean Doppler vs. Spectral Width
Notes
•+/- Asymmetry
• Faster + wider are correlated
• Narrow, fast population
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October 15, 2003 21Radar RemoteSensing Laboratory
University of Washington
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?)
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October 15, 2003 22Radar RemoteSensing Laboratory
University of Washington
Speed-up at Far Ranges:Individual Cases
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October 15, 2003 23Radar RemoteSensing Laboratory
University of Washington
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!)
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October 15, 2003 24Radar RemoteSensing Laboratory
University of Washington
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.
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October 15, 2003 25Radar RemoteSensing Laboratory
University of Washington
Other Features in Our Data…
• Narrow, fast population: Examples• Often see spectra with 2nd, faster peak• Associated with fine range structure.
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October 15, 2003 26Radar RemoteSensing Laboratory
University of Washington
Other Features in Our Data…
A shear in velocity / electric field over range
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October 15, 2003 27Radar RemoteSensing Laboratory
University of Washington
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.