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STAMMS Conference Meeting, Orleans, France
May 2003
R. L. Mutel, D. A. Gurnett, I. Christopher, M. Schlax
University of Iowa
Spatial and Temporal Properties of AKR Burst Emission Derived From WBD VLBI
Studies
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STAMMS Meeting, Orleans, May 2003
University of Iowa Wideband Data Plasma Wave Instrument (WBD)
• Identical WBD instruments are mounted on all four spacecraft. Single dipole antenna used.
• Real-time downlink of 220 kb/s to the NASA Deep Space Network (DSN). (One DSN antenna per S/C!)
• DSN provides real-time time stamps (accuracy 10 s).
• AKR studies use 125, 250, and 500 KHz bands, 10 KHZ bandwidth, 37 s sampling time.
• High frequency/time resolution capability of WBD is the primary characteristic that makes WBD unique from the other Cluster wave experiments, which operate at much lower data rates.
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STAMMS Meeting, Orleans, May 2003
Example of WBD Dynamic Spectra
(250-262 KHz, 30 sec), S/C separation ~300 km
Auroral Kilometric Radiation (AKR) Bursts
Auroral Kilometric Radiation (AKR) Bursts
Spacecraft
1
3
4
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STAMMS Meeting, Orleans, May 2003
Example of shadowing by plasmasphere at low magnetic
latitude (15 May 2001)
m = -53°
m = -38°
m = -32°
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STAMMS Meeting, Orleans, May 2003
Dynamic Spectra of Common AKR Bursts
AKR bursts have 100-400 KHz bandwdth
(courtesy R. Anderson, GEOTAIL)
Cluster spectra
(10 KHz bandwidth)
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STAMMS Meeting, Orleans, May 2003
VLBI Source Location Algorithm: Differential delay measurement
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STAMMS Meeting, Orleans, May 2003
Sample Dynamic Spectrum, Waveform and Cross-correlation
Waveforms from each Cluster WBD receiver for AKR burst shown
at leftPeak is fit with Gaussian, delay uncertainty ~ 0.3 ms
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STAMMS Meeting, Orleans, May 2003
• A uniform 3-d grid of points is constructed centered on the Earth with spacing 0.1 Re and dimension 8 Re on each side (512,000 pts).
• The propagation time to each satellite is computed from each grid point.
• Differential delays are then computed for each baseline and compared with the observed delays, as measured by cross-correlating the waveforms from each pair of spacecraft
AKR Burst Position search algorithm
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STAMMS Meeting, Orleans, May 2003
VLBI position uncertainty calculation
1 12 22 2
2 21
2 2
B Bz x z x
c
500z
x c kmB
2
2 5,000z
x c kmB
Delay uncertainties in plane and parallel to line of sight:
Typical uncertainty in plane:
Typical uncertainty in plane:x
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STAMMS Meeting, Orleans, May 2003
Uncertainty mapped to Earth (CGM coordinates)
Uncertainty ~ 500 km -1000 km
Uncertainty ~ 200 km -400 km
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STAMMS Meeting, Orleans, May 2003
Refractive effects
effect on AKR burst location determination
unimportant for S/C magnetic latitudes > 40°
(plasmasphere model
Gallagher et al.2000)
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STAMMS Meeting, Orleans, May 2003
Refractive Ray tracing corrections
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STAMMS Meeting, Orleans, May 2003
AKR Bursts: Locus of Allowed Locations
Locus of allowed locations for AKR burst on 10 July 2002 at 08:47:02 and illustrated at right.
The top panels show the unconstrained solution of all allowed points (left is oblique view; right view is from spacecraft).
The lower panel shows the constrained solution assuming the AKR emission arises from a radius distance from Earth consistent with the observed frequency being identified with the electron gyrofrequency. A model auroral oval is shown for reference.
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STAMMS Meeting, Orleans, May 2003
AKR Burst Locations: The movie
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STAMMS Meeting, Orleans, May 2003
Summary of 4 Spacecraft VLBI Epochs (Fully Analyzed)Southern Hemisphere Observations
Northern Hemisphere Observations
Epoch N /S S/C DOY UT N MLT Comments
20jul02 N 43/28 02-201 13.0-15.0 34 -4.5 62.7 Very spread in inv. Lat.
28oct02 N 32 02-301 10.6-11.1 150 -2.3 63.8 Well defined 09nov02 N 54/34 02-313 6.5-8.4 568 -3.3 68.4 Beautiful, well
defined 14nov02 N 19 02-318 7.0-8.5 130 2.6 74.8 Well defined 15dec02 N 34 02-349 3.4-3.7 22 1.7 70.6 22dec02 N 34 02-356 4.4-4.7 277 -0.3 68.0 29dec02 N 47 02-363 5.0-6.5 221 -4.7 70.2 Well defined 22jan03 N 52/34 03-022 0.9-2.7 372 -6.8 71.3 Temporal
migration Total/average 9 1774 -2.2 68.7
Epoch N /S S/C DOY UT N MLT Comments 20jun 02 S -25/-35 02-171 16.1-16.2 56 -8.5 -75.5 Well defined 05jul02 S -55 02-186 11.6-12.4 84 1.3 -70.5 Along line 10jul02 S -55/-70 02-191 8.2-10.0 319 0.8 -75.1 Well defined 17jul02 S -45/-60 02-198 10.6-11.9 81 2.1 -71.7 Along line, mostly
A zone 10aug02 S -60/-73 02-222 7.4-8.5 83 -1.0 -70.6 Very elongated
along line 19aug02 S -66/-76 02-231 17.2-19.5 171 -1.3 -75.0 31aug02 S -70/-80 02-243 16.0-19.5 242 -1.6 -77.8 26sep02 S -62 02-269 18.1-18.9 34 -2.6 -74.3
Total/average 9 1070 -1.35 -73.8
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STAMMS Meeting, Orleans, May 2003
Histogram of AKR Burst Locations
CGM coordinates, 5 epochs Southern hemisphere only
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STAMMS Meeting, Orleans, May 2003
November 9 Locations: Varying Perspectives (Animation)
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STAMMS Meeting, Orleans, May 2003
Nov 9 :The Movie Mapped onto CGM coordinates
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STAMMS Meeting, Orleans, May 2003
Observed distribution of AKR bursts
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STAMMS Meeting, Orleans, May 2003
AKR Burst locations
vs.
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STAMMS Meeting, Orleans, May 2003
Example of position uncertainty including
depth-of-field(9 Oct 02)
Blue: fgyro – 10%
Red: fgyro + 10%
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STAMMS Meeting, Orleans, May 2003
Example of AKR Burst location with
Uncertainties projected into
100km Altitude, CGM coordinates
(29 Dec 02)
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STAMMS Meeting, Orleans, May 2003
Polar Average Images of Northern Auroral by month (Liou et al. 1997)
Evening Peak ~22h MLT April -May
Day peak at ~15h MLT June -July
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STAMMS Meeting, Orleans, May 2003
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STAMMS Meeting, Orleans, May 2003
AKR burst mean location drift: example119 Aug 2002,
Southern hemisphere
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STAMMS Meeting, Orleans, May 2003
AKR burst mean location drift: example222 Jan 2003, N hemisphere