Spatial and Temporal Properties of AKR Burst
Emission Derived From WBD VLBI Studies
R. L. Mutel, D. A. Gurnett, I. Christopher, M. Schlax
University of Iowa
STAMMS Conference Meeting,
Orleans, France
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.
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
Spacecraft
1
3
4
STAMMS Meeting, Orleans, May 2003
Example of shadowing by
plasmasphere at low magnetic
latitude (15 May 2001)
m = -53°
m = -38°
m = -32°
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)
STAMMS Meeting, Orleans, May 2003
VLBI Source Location Algorithm:
Differential delay measurement
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Sample Dynamic Spectrum, Waveform and Cross-correlation
Waveforms from each
Cluster WBD receiver for
AKR burst shown at left
Peak is fit with Gaussian, delay
uncertainty  ~ 0.3 ms
STAMMS Meeting, Orleans, May 2003
AKR Burst Position search algorithm
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 crosscorrelating the waveforms from
each pair of spacecraft
STAMMS Meeting, Orleans, May 2003
VLBI position uncertainty calculation
Delay uncertainties in plane  and
parallel to line of sight:
1
1

2 2
2 2
1 
B
  
B
  
     z 2     x     z 2     x   
c  
2
  
2
  


Typical uncertainty in  plane:
z
 x     c  500 km
B
Typical uncertainty in
x
plane:
2
z
 x  2    c  5,000 km
B
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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Refractive effects
effect on AKR burst
location determination
unimportant for S/C
magnetic latitudes > 40°
(plasmasphere model
Gallagher et al.2000)
STAMMS Meeting, Orleans, May 2003
Refractive Ray tracing corrections
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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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AKR Burst Locations: The movie
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Summary of 4 Spacecraft VLBI Epochs (Fully Analyzed)
Southern Hemisphere Observations
Epoch
20jun 02
05jul02
10jul02
17jul02
N /S
S
S
S
S
S/C
-25/-35
-55
-55/-70
-45/-60
DOY
02-171
02-186
02-191
02-198
UT
16.1-16.2
11.6-12.4
8.2-10.0
10.6-11.9
N
56
84
319
81
MLT
-8.5
1.3
0.8
2.1

-75.5
-70.5
-75.1
-71.7
10aug02
S
-60/-73
02-222
7.4-8.5
83
-1.0
-70.6
19aug02
31aug02
26sep02
Total/average
S
S
S
9
-66/-76
-70/-80
-62
02-231
02-243
02-269
17.2-19.5
16.0-19.5
18.1-18.9
171
242
34
1070
-1.3
-1.6
-2.6
-1.35
-75.0
-77.8
-74.3
-73.8
Comments
Well defined
Along line
Well defined
Along line, mostly
A zone
Very elongated
along line
Northern Hemisphere Observations
Epoch
20jul02
N /S
N
S/C
43/28
DOY
02-201
UT
13.0-15.0
N
34
MLT
-4.5

62.7
28oct02
09nov02
N
N
32
54/34
02-301
02-313
10.6-11.1
6.5-8.4
150
568
-2.3
-3.3
63.8
68.4
14nov02
15dec02
22dec02
29dec02
22jan03
N
N
N
N
N
19
34
34
47
52/34
02-318
02-349
02-356
02-363
03-022
7.0-8.5
3.4-3.7
4.4-4.7
5.0-6.5
0.9-2.7
130
22
277
221
372
2.6
1.7
-0.3
-4.7
-6.8
74.8
70.6
68.0
70.2
71.3
Total/average
9
STAMMS Meeting, Orleans,1774
May 2003 -2.2
68.7
Comments
Very spread in inv.
Lat.
Well defined
Beautiful, well
defined
Well defined
Well defined
Temporal
migration
Histogram of AKR Burst
Locations
CGM coordinates, 5 epochs
Southern hemisphere only
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November 9 Locations: Varying Perspectives (Animation)
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Nov 9 :The Movie
Mapped onto CGM coordinates
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Observed distribution of AKR bursts
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AKR Burst locations
vs.
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Example of position
uncertainty including
depth-of-field
(9 Oct 02)
Blue: fgyro – 10%
Red: fgyro + 10%
STAMMS Meeting, Orleans, May 2003
Example of AKR
Burst location with
Uncertainties
projected into
100km Altitude,
CGM coordinates
(29 Dec 02)
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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AKR burst mean location drift: example1
19 Aug 2002,
Southern hemisphere
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AKR burst mean location drift: example2
22 Jan 2003, N hemisphere
STAMMS Meeting, Orleans, May 2003