FAST Observations
of Wave Packets
in the AKR Source Region
R. J. Strangeway, P. L. Pritchett
University of California, Los Angeles
R. E. Ergun, C. W. Carlson,
J. P. McFadden, G. T. Delory
University of California, Berkeley
Outline
The accelerated electrons as the free energy source for AKR.
Pumping of AKR by parallel electric field.
Plasma Wave Tracker Waveforms. Types of fine structure
observed.
Does FAST observe narrow (few Hz) tones?
Conclusions
Long narrow tones are a consequence of propagation and
refraction.
Burstiness may be a signature of the reformation of the
distribution.
Free Energy Source for AKR
FAST has shown that the accelerated electrons can be the major
free energy source for AKR.
Within the AKR source region the electron density is dominated by
hot electrons [Strangeway et al., GRL, 1998] and the waves can lie
below the electron gyro-frequency, with electric fields
perpendicular to the ambient field [Ergun et al., GRL, 1998;
Delory et al., GRL 1998].
In addition, the wave magnetic field is polarized parallel to the
ambient field.
This is consistent with perpendicularly propagating X-mode
waves. Such waves can resonate with energized electrons.
Because the parallel electric field continually re-energizes the
electron distribution, AKR can be generated over a large range of
altitudes and presumably to high intensity. This point was also
made by Louarn et al. [JGR, 1990].
FAST Orbit 1761
800
-8
500
400
300
800
-14
-6
(nT)2/Hz
600
500
Fri Dec 4 21:23:16 1998
(kHz)
AC Mag 21"
700
400
UT
ALT
ILAT
MLT
300
:44:20
4153.5
69.7
22.2
:44:30
4152.5
69.9
22.2
:44:40
:44:50
:45:00
4151.3
4150.2
4148.9
70.1
70.3
70.5
22.2
22.2
22.2
Minutes from 1997-01-31/06:44:20
:45:10
4147.5
70.8
22.2
:45:20
4146.1
71.0
22.2
-10
(V/m)2/Hz
600
log
(kHz)
AC E 55m
700
Electron Distribution in
Density Cavity
1•10 5
Perp. Velocity (km/s)
5•104
-13.4
2
3
Loss Cone
1
0
-14.9
-5•104
-16.3
Upgoing to
Magnetosphere
-1•10 5
-1•10 5
Downgoing to
Ionosphere
-5•10 4
0
Para. Velocity (km/s)
5•10 4
Energy Flow
1. Acceleration by Electric Field
2. Mirroring by Magnetic Mirror
3. Diffusion through Auroral Kilometric Radiation
1•10 5
-17.8
Log10 (Phase Space Density)
-12.0
Is Feedback Necessary to Generate AKR?
Because the parallel electric field acts as a pump for AKR
generation, do we need to invoke feedback as a means for
enhancing growth?
Calvert [JGR, 1982] suggests that reflections at the edges of the
AKR generation region could allow multiple transitions of the
source region – a positive feedback loop. Such feedback could
result in high intensities and narrow bandwidths (very narrow,
Calvert [1982] requires other factors to broaden the wave
spectrum). Such feedback loops may be susceptible to triggering,
explaining the apparent triggering of AKR by Type III bursts
[Calvert, GRL, 1981].
Baumback and Calvert [GRL, 1987] provide evidence of narrow
tones ~ 5 Hz bandwidth. Does FAST see such tones?
Types of Fine Structure
Long relatively narrow tones – closest to Baumback and Calvert
observations. Identification criteria: typically change by less than 1
kHz/s, continue for several seconds, narrow (few 100 Hz).
Short risers and fallers.
Identification criteria: change by less than 5 kHz/s, short duration,
often repeat.
Bursty – packet like.
Identification criteria: “Speckled” spectrogram.
Statistics:
Scanned 50 Orbits.
13 events in source region (PWT band close to model field
electron gyro frequency).
22 events not in source region.
FAST Orbit 1752
800
-6
log
(V/m)2/Hz
log
(V/m)2/Hz
(kHz)
AC E 55m
700
600
500
400
300
450
-12
-5
440
(kHz)
PWT E
430
420
Fri Dec 4 18:56:23 1998
410
400
390
UT
ALT
ILAT
MLT
:50:30
4102.8
69.8
22.1
53 s
-11
:50:40
:50:50
:51:00
:51:10
4099.8
4096.7
4093.5
4090.2
70.0
70.3
70.5
70.7
22.1
22.1
22.1
22.1
Minutes from 1997-01-30/10:50:30
FAST Orbit 1752
PWT - Wave Form
30
10
0
-10
-20
-30
:53.5
4095.6
70.3
22.1
UT
ALT
ILAT
MLT
Fri Dec 4 10:54:56 1998
(mV/m)
20
2.5 s
:54.0
:54.5
:55.0
:55.5
4095.4
4095.2
4095.1
4094.9
70.4
70.4
70.4
70.4
22.1
22.1
22.1
22.1
Seconds from 1997-01-30/10:50:53
FAST Orbit 1752
PWT - Wave Form
30
10
0
-10
-20
-30
:54.25
4095.3
70.4
22.1
UT
ALT
ILAT
MLT
Fri Dec 4 10:59:27 1998
(mV/m)
20
0.2 s
:54.30
:54.35
:54.40
4095.3
4095.3
4095.3
70.4
70.4
70.4
22.1
22.1
22.1
Seconds from 1997-01-30/10:50:54
FAST Orbit 1752
PWT - Wave Form
30
UT
ALT
ILAT
MLT
10
0
Fri Dec 4 11:06:10 1998
(mV/m)
20
-10
-20
-30
:54.340
4095.3
70.4
22.1
:54.342
:54.344
:54.346
:54.348
4095.3
4095.3
4095.3
4095.3
70.4
70.4
70.4
70.4
22.1
22.1
22.1
22.1
Seconds from 1997-01-30/10:50:54
:54.350
4095.3
70.4
22.1
10 ms
FAST Orbit 1779
800
-6
log
600
500
(V/m)2/Hz
(kHz)
AC E 55m
700
400
300
360
-12
-5
355
log
345
Fri Dec 4 18:36:30 1998
340
335
330
:44:30
4105.8
68.0
23.4
UT
ALT
ILAT
MLT
:44:35
4104.4
68.1
23.5
:44:40
:44:45
:44:50
4102.9
4101.4
4099.8
68.2
68.3
68.3
23.5
23.5
23.5
Minutes from 1997-02-01/22:44:30
(V/m)2/Hz
(kHz)
PWT E
350
28 s
-11
:44:55
4098.3
68.4
23.5
FAST Orbit 1779
20
0
Fri Dec 4 10:04:12 1998
(mV/m)
PWT - Wave Form
40
-20
-40
UT
ALT
ILAT
MLT
:43.0
4102.0
68.2
23.5
:43.5
:44.0
:44.5
4101.8
4101.7
4101.5
68.2
68.2
68.2
23.5
23.5
23.5
Seconds from 1997-02-01/22:44:43
2.5 s
:45.0
4101.4
68.3
23.5
FAST Orbit 1779
20
0
Fri Dec 4 10:10:16 1998
(mV/m)
PWT - Wave Form
40
-20
-40
UT
ALT
ILAT
MLT
:43.60
4101.8
68.2
23.5
0.2 s
:43.65
:43.70
:43.75
4101.8
4101.8
4101.7
68.2
68.2
68.2
23.5
23.5
23.5
Seconds from 1997-02-01/22:44:43
FAST Orbit 1779
20
0
Fri Dec 4 10:12:55 1998
(mV/m)
PWT - Wave Form
40
-20
-40
UT
ALT
ILAT
MLT
:43.690
4101.8
68.2
23.5
:43.695
:43.700
:43.705
4101.8
4101.8
4101.8
68.2
68.2
68.2
23.5
23.5
23.5
Seconds from 1997-02-01/22:44:43
:43.710
4101.8
68.2
23.5
20 ms
FAST Orbit 1768
600
-8
550
log
(V/m)2/Hz
AKR E 55m
(kHz)
500
450
400
350
300
360
-14
-5
355
log
(V/m)2/Hz
PWT E
(kHz)
350
345
20 s
Wed Aug 12 11:31:36 1998
340
335
330
UT
ALT
ILAT
MLT
:19:55
4111.0
67.9
23.6
-11
:20:00
:20:05
:20:10
4109.6
4108.2
4106.7
68.0
68.1
68.2
23.6
23.6
23.6
Minutes from 1997-01-31/22:19:55
FAST Orbit 1768
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Wed Aug 12 16:18:29 1998
(mV/m)
50
0
-50
-100
UT
ALT
ILAT
MLT
:01.0
4109.3
68.0
23.6
2.5 s
:01.5
:02.0
:02.5
4109.2
4109.0
4108.9
68.0
68.0
68.0
23.6
23.6
23.6
Seconds from 1997-01-31/22:20:01
FAST Orbit 1768
100
Wed Aug 12 16:21:15 1998
(mV/m)
50
0
-50
-100
UT
ALT
ILAT
MLT
:01.450
4109.2
68.0
23.6
:01.475
:01.500
:01.525
4109.2
4109.2
4109.2
68.0
68.0
68.0
23.6
23.6
23.6
Seconds from 1997-01-31/22:20:01
0.1 s
:01.550
4109.2
68.0
23.6
FAST Orbit 1768
100
0
-50
-100
UT
ALT
ILAT
MLT
Wed Aug 12 16:22:24 1998
(mV/m)
50
:01.486 :01.488 :01.490 :01.492 :01.494 :01.496 :01.498
4109.2 4109.2 4109.2 4109.2 4109.2 4109.2 4109.2
68.0
68.0
68.0
68.0
68.0
68.0
68.0
23.6
23.6
23.6
23.6
23.6
23.6
23.6
Seconds from 1997-01-31/22:20:01
12 ms
Source Region
Number
13
long-tone
3(1)
23%(8%)
fallers/risers
7
54%
bursty
9
69%
Non-Source Region
Number
22
long-tone fallers/risers
17(16)
10(8)
77%(73%) 15%(36%)
Numbers in parentheses correspond to clear signals.
bursty
10
45%
Does FAST Observe Narrow Tones?
FFT’s shown earlier have 64 Hz frequency resolution – too broad
to resolve 5 Hz.
How do Baumback and Calvert [1987] resolve 5 Hz? For example
a tone varying by 400 Hz/s should not be resolvable to less than 20
Hz ( ∆f / ∆t ≈ 1) .
Baumback and Calvert used filtered auto-correlation:
n
f c (t ,τ ) =
∑ W ( kδt ) f (t − kδt + τ / 2) f (t − kδt − τ / 2)
k =0
n
∑ W ( kδt )
k =0
τ up to 85.5 ms, implies ∆f ≈ 40 Hz but W is a filter of width ≈ 14
ms, implies ∆f ≈ 6 Hz.
Is this actual bandwidth of the signal? Or is it a feature of the
filtering?
FAST Orbit 1752
445
-5
log
(V/m)2/Hz
PWT E
(kHz)
440
435
430
-11
-5
440
log
(V/m)2/Hz
PWT E (Auto_n)
(kHz)
425
445
435
430
-11
1.00
0.10
Fri Dec 4 13:29:23 1998
Peak Width (FFT)
(kHz)
425
10.00
0.01
UT
ALT
ILAT
MLT
:53.5
4095.6
70.3
22.1
:54.0
:54.5
:55.0
4095.4
4095.2
4095.1
70.4
70.4
70.4
22.1
22.1
22.1
Seconds from 1997-01-30/10:50:53
:55.5
4094.9
70.4
22.1
FAST Orbit 1779
360
-5
355
log
(V/m)2/Hz
PWT E
(kHz)
350
345
340
335
330
360
-11
-5
350
log
(V/m)2/Hz
PWT E (Auto_n)
(kHz)
355
345
340
335
-11
1.00
0.10
Fri Dec 4 18:44:32 1998
Peak Width (FFT)
(kHz)
330
10.00
0.01
UT
ALT
ILAT
MLT
:43.0
4102.0
68.2
23.5
:43.5
:44.0
:44.5
4101.8
4101.7
4101.5
68.2
68.2
68.2
23.5
23.5
23.5
Seconds from 1997-02-01/22:44:43
:45.0
4101.4
68.3
23.5
FAST Orbit 1768
360
-5
log
(V/m)2/Hz
PWT E
(kHz)
355
350
345
-11
-5
355
log
(V/m)2/Hz
PWT E (Auto_n)
(kHz)
340
360
350
345
-11
1.00
0.10
Fri Dec 4 13:02:32 1998
Peak Width (FFT)
(kHz)
340
10.00
0.01
UT
ALT
ILAT
MLT
:01.0
4109.3
68.0
23.6
:01.5
:02.0
:02.5
4109.2
4109.0
4108.9
68.0
68.0
68.0
23.6
23.6
23.6
Seconds from 1997-01-31/22:20:01
Summary
Using filtered auto-correlation does result in narrow tones
(~100 Hz) – but not clear if this is a consequence of the filtering
itself.
In addition, long tones are observed away from the source region.
This implies propagation and refraction causes narrower more
apparently continuous tones.
Risers/fallers are observed both in the source region and away
from the source. Probably due to modifications of the source (e.g.
ion beams density fluctuations). Note ∂f / ∂t = 40 kHz/s
corresponds to v ≈ 400 km/s for 4000 km altitude, fce = 300 kHz.
Burstiness could be a signature of reformation of the electron
distribution. Test by simulations.