Fine Structure in Auroral Kilometric Radiation:
Evidence for Electromagnetic and Electrostatic Ion
Cyclotron Waves
R. L. Mutel (& D. Menietti)
University of Iowa
Astrophysics Seminar
October 2004
Earth as a radio source
UI Space/Astro Seminar October 2004
AKR is generated 6000-12000 km
above the visible aurora
AKR is generated at f = ωce
by electron cyclotron maser
UI Space/Astro Seminar October 2004
Dynamic Spectra of Common AKR Bursts
Geotail
Cluster
UI Space/Astro Seminar October 2004
May 15 AKR Event: Variation with Magnetic Latitude
m = -53°
m = -38°
m = -32°
UI Space/Astro Seminar October 2004
Example of WBD Dynamic Spectra
(250-262 KHz, 30 sec), S/C separation ~300 km
Auroral Kilometric
Radiation (AKR)
Bursts
Spacecraft
1
3
4
UI Space/Astro Seminar October 2004
FAST Observations of AKR Source Region (Ergun et al. Ap. J. 538, 456)
Note e- depletion
in auroral cavity
UI Space/Astro Seminar October 2004
FAST Observations of AKR Source Region (Ergun et al. Ap. J. 538, 456)
Shell instability
UI Space/Astro Seminar October 2004
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.
UI Space/Astro Seminar October 2004
VLBI Source Location Algorithm:
Differential delay measurement
UI Space/Astro Seminar October 2004
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
UI Space/Astro Seminar October 2004
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
UI Space/Astro Seminar October 2004
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
UI Space/Astro Seminar October 2004
Uncertainty mapped to Earth
(CGM coordinates)
Uncertainty ~ 500 km -1000 km
Uncertainty ~ 200 km -400 km
UI Space/Astro Seminar October 2004
Refractive effects
effect on AKR burst
location determination
unimportant for S/C
magnetic latitudes > 40°
(plasmasphere model
Gallagher et al.2000)
UI Space/Astro Seminar October 2004
Refractive Ray tracing corrections
UI Space/Astro Seminar October 2004
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.
UI Space/Astro Seminar October 2004
AKR Burst Locations: The movie
UI Space/Astro Seminar October 2004
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
UI Space/Astro Seminar October
17742004 -2.2
68.7
Comments
Very spread in inv.
Lat.
Well defined
Beautiful, well
defined
Well defined
Well defined
Temporal
migration
November 9 Locations: Varying Perspectives (Animation)
UI Space/Astro Seminar October 2004
Nov 9 :The Movie
Mapped onto CGM coordinates
UI Space/Astro Seminar October 2004
Observed distribution of AKR bursts
UI Space/Astro Seminar October 2004
AKR Burst locations
vs.
UI Space/Astro Seminar October 2004
Example of position
uncertainty including
depth-of-field
(9 Oct 02)
Blue: fgyro – 10%
Red: fgyro + 10%
UI Space/Astro Seminar October 2004
Example of AKR
Burst location with
Uncertainties
projected into
100km Altitude,
CGM coordinates
(29 Dec 02)
UI Space/Astro Seminar October 2004
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
UI Space/Astro Seminar October 2004
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
UI Space/Astro Seminar October
17742004 -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
UI Space/Astro Seminar October 2004
November 9 Locations: Varying Perspectives (Animation)
UI Space/Astro Seminar October 2004
Nov 9 :The Movie
Mapped onto CGM coordinates
UI Space/Astro Seminar October 2004
Observed distribution of AKR bursts
UI Space/Astro Seminar October 2004
AKR Burst locations
vs.
UI Space/Astro Seminar October 2004
Example of position
uncertainty including
depth-of-field
(9 Oct 02)
Blue: fgyro – 10%
Red: fgyro + 10%
UI Space/Astro Seminar October 2004
Example of AKR
Burst location with
Uncertainties
projected into
100km Altitude,
CGM coordinates
(29 Dec 02)
UI Space/Astro Seminar October 2004
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
UI Space/Astro Seminar October 2004
UI Space/Astro Seminar October 2004
AKR burst mean location drift: example1
19 Aug 2002,
Southern hemisphere
UI Space/Astro Seminar October 2004
AKR burst mean location drift: example2
22 Jan 2003, N hemisphere
UI Space/Astro Seminar October 2004
First simultaneous AKR/VLBI location map with UV
image (IMAGE). June 8 , 2004
AKR burst is
associated with
discrete auroral
arc
UI Space/Astro Seminar October 2004
‘Rain’ AKR bursts: Narrow, rapidly drifting structures
“Rain” AKR
bursts
“normal”
AKR burst
UI Space/Astro Seminar October 2004
AKR Dynamic Spectrum 125 KHz
17 July 2002 11:42:30 – 11:43:00
Slope -8.7 KHz/sec
UI Space/Astro Seminar October 2004
AKR Dynamic Spectrum 125 KHz
17 July 2002 11:42:30 – 11:43:00
Slope = -3.1 KHz/sec
UI Space/Astro Seminar October 2004
Rain AKR bursts at 125 KHz
31Aug 2002 16:14:30-16:15:00 UT
Slope -6.3 KHz/sec
Modulated periodic
structures
UI Space/Astro Seminar October 2004
Rain AKR bursts at 500 KHz
31Aug 2002 19:26:00- 19:26:30 UT
Slope -12.5 KHz/sec
UI Space/Astro Seminar October 2004
Derived Speed compared with Alfven, and Electron, Ion Acoustic
speeds versus Radial distance (assumes 0.1-10 keV particles)
Alfven speed
Electron acoustic speed
500 KHz
250 KHz
125 KHz
Ion acoustic speed
UI Space/Astro Seminar October 2004
Derived Exciter speed along B field
125 KHz
500 KHz
UI Space/Astro Seminar October 2004
Exitor speed derived from frequency drift
1. Assume EM at electron gyro-frequency, dipolar magnetic field
f obs
 re 
 f0  
r
3
f o  1.66 MHz
2. Use (negative) frequency drift to derive (upward) wave3 speed
r
df df dr
df
   V   3V  e4
dt dr dt
dr
r
3. Recast in terms of fobs
V  f obs   
re df  f o 


f 0 dt  f obs 

2
3
4. Amplitude modulation conversion to spatial wavelength
f obs
df
 3
 25 Hz / km
dt
r
UI Space/Astro Seminar October 2004
Striated AKR from 0-120 KHz
(Menietti et al. 2000)
Slope ~ 1 KHz/sec @100 KHz
Slope ~ 0.5 KHz/sec @40 KHz
UI Space/Astro Seminar October 2004
UI Space/Astro Seminar October 2004
Modulation of Striated AKR: Is it due
to Faraday rotation?
  c 2   ne  B dl
L
Requires:
• Linearly polarized emission (but AKR is circularly polarized)
• Δ ≈ 3 turns/KHz @ 125 KHz (λ=2.4 km) =>
RM = 10-3 rad-m-2
• This may be plausible: B ~ 0.1 gauss, ne ~ 10 cm-3, L ~ 100 km
UI Space/Astro Seminar October 2004
UI Space/Astro Seminar October 2004
Sampled at 1024 channels
UI Space/Astro Seminar October 2004
Sampled at 2048 channels
UI Space/Astro Seminar October 2004
Wavelet dynamic
spectrum of Striated
bursts, 2 spacecraft
correlation
UI Space/Astro Seminar October 2004
Electromagnetic Ion-Cyclotron Wave Group Speed
vs. Frequency
d

dk
2cN  ci    
 pi
2
 ci
 pe 2 ci    
 2







pi
ce
 ci

where

 pe
  pi
N 




ce

 ci
2
2





1
2
Note: Plot assumes
• Dipolar B field
•Ne (r) ~ r -3.2 (Persoon model)
UI Space/Astro Seminar October 2004
Banded Modulation of AKR: Evidence for
Electrostatic Ion Cyclotron (EIC) Waves?
UI Space/Astro Seminar October 2004
Close-up of AKR banded modulation
ci  140 Hz
Separation = 120 ± 5 Hz
UI Space/Astro Seminar October 2004
UI Space/Astro Seminar October 2004
 
UI Space/Astro Seminar October 2004
  2
What excites rain AKR?
Model assumptions
• Observed emission is same as ‘normal’ AKR: electron
cyclotron maser near electron cyclotron frequency
• Earth’s magnetic field is strictly dipolar (OK for r < 2.5 Re)
• A disturbance traveling parallel to B stimulates EM emission
• Observed slope indicates phase speed of wave along B field
• Observed banded modulation may indicates characteristic
wavelength of exciter (after conversion to spatial separation)
UI Space/Astro Seminar October 2004