Astrophysics Seminar October 2004 R. L. Mutel (& D. Menietti) University of Iowa Fine Structure in...

Astrophysics Seminar

October 2004

R. L. Mutel (& D. Menietti)

University of Iowa

Fine Structure in Auroral Kilometric Radiation: Evidence for

Electromagnetic and Electrostatic Ion Cyclotron Waves

UI Space/Astro Seminar October 2004

Earth as a radio source


AKR is generated 6000-12000 km above the visible aurora

AKR is generated at f = ωce

by electron cyclotron maser


Dynamic Spectra of Common AKR Bursts

Geotail

Cluster


May 15 AKR Event: Variation with Magnetic Latitude

m = -53°

m = -38°

m = -32°


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


FAST Observations of AKR Source Region (Ergun et al. Ap. J. 538, 456)

Note e- depletion in auroral cavity

Note e- depletion in auroral cavity


FAST Observations of AKR Source Region (Ergun et al. Ap. J. 538, 456)

Shell instabilityShell instability


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.


VLBI Source Location Algorithm: Differential delay measurement


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


• 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


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


Uncertainty mapped to Earth (CGM coordinates)

Uncertainty ~ 500 km -1000 km

Uncertainty ~ 200 km -400 km


Refractive effects

effect on AKR burst location determination

unimportant for S/C magnetic latitudes > 40°

(plasmasphere model

Gallagher et al.2000)


Refractive Ray tracing corrections


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.


AKR Burst Locations: The movie


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


November 9 Locations: Varying Perspectives (Animation)


Nov 9 :The Movie Mapped onto CGM coordinates


Observed distribution of AKR bursts


AKR Burst locations

vs.


Example of position uncertainty including

depth-of-field(9 Oct 02)

Blue: fgyro – 10%

Red: fgyro + 10%


Example of AKR Burst location with

Uncertainties projected into

100km Altitude, CGM coordinates

(29 Dec 02)


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


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


Histogram of AKR Burst Locations

CGM coordinates, 5 epochs Southern hemisphere only


November 9 Locations: Varying Perspectives (Animation)


Nov 9 :The Movie Mapped onto CGM coordinates


Observed distribution of AKR bursts


AKR Burst locations

vs.


Example of position uncertainty including

depth-of-field(9 Oct 02)

Blue: fgyro – 10%

Red: fgyro + 10%


Example of AKR Burst location with

Uncertainties projected into

100km Altitude, CGM coordinates

(29 Dec 02)


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


AKR burst mean location drift: example119 Aug 2002,

Southern hemisphere


AKR burst mean location drift: example222 Jan 2003, N hemisphere


First simultaneous AKR/VLBI location map with UV image (IMAGE). June 8 , 2004

AKR burst is associated with discrete auroral

arc


‘Rain’ AKR bursts: Narrow, rapidly drifting structures

“normal” AKR burst

“Rain” AKR bursts


AKR Dynamic Spectrum 125 KHz17 July 2002 11:42:30 – 11:43:00

Slope -8.7 KHz/sec


AKR Dynamic Spectrum 125 KHz17 July 2002 11:42:30 – 11:43:00

Slope = -3.1 KHz/sec


Rain AKR bursts at 125 KHz31Aug 2002 16:14:30-16:15:00 UT

Modulated periodic structures

Slope -6.3 KHz/sec


Rain AKR bursts at 500 KHz31Aug 2002 19:26:00- 19:26:30 UT

Slope -12.5 KHz/sec


Derived Speed compared with Alfven, and Electron, Ion Acoustic speeds versus Radial distance (assumes 0.1-10

keV particles)

Ion acoustic speed

Electron acoustic speed

Alfven speed

500 KHz 250 KHz 125 KHz


Derived Exciter speed along B field

125 KHz

500 KHz


Exitor speed derived from frequency drift

3

43 erdf df dr df

V Vdt dr dt dr r

2

3

0

e oobs

obs

r fdfV f

f dt f

3

0 1.66eobs o

rf f f MHz

r

2. Use (negative) frequency drift to derive (upward) wave speed

1. Assume EM at electron gyro-frequency, dipolar magnetic field

3. Recast in terms of fobs

4. Amplitude modulation conversion to spatial wavelength

3 25 /obsfdfHz km

dt r


Striated AKR from 0-120 KHz(Menietti et al. 2000)

Slope ~ 1 KHz/sec @100 KHz

Slope ~ 0.5 KHz/sec @40 KHz


Modulation of Striated AKR: Is it due to Faraday rotation?

2e

L

c n B dl

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


Sampled at 1024 channels


Sampled at 2048 channels


Wavelet dynamic spectrum of Striated bursts, 2 spacecraft

correlation


Electromagnetic Ion-Cyclotron Wave Group Speed vs. Frequency

2

22

2 ci

pe cicipi

ci pi ce

cNd

dk

12 2 2

pi pe

ci ce

N

where

Note: Plot assumes

• Dipolar B field

•Ne (r) ~ r -3.2 (Persoon model)


Banded Modulation of AKR: Evidence for Electrostatic Ion Cyclotron (EIC) Waves?


Close-up of AKR banded modulation

140ci Hz

Separation = 120 ± 5 Hz


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)

Astrophysics Seminar October 2004 R. L. Mutel (& D. Menietti) University of Iowa Fine Structure in...

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