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

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.

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

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

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

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

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

STAMMS Meeting, Orleans, May 2003

Uncertainty mapped to Earth (CGM coordinates)

Uncertainty ~ 500 km -1000 km

Uncertainty ~ 200 km -400 km

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)

STAMMS Meeting, Orleans, May 2003

Refractive Ray tracing corrections

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.

STAMMS Meeting, Orleans, May 2003

AKR Burst Locations: The movie

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

STAMMS Meeting, Orleans, May 2003

Histogram of AKR Burst Locations

CGM coordinates, 5 epochs Southern hemisphere only

STAMMS Meeting, Orleans, May 2003

November 9 Locations: Varying Perspectives (Animation)

STAMMS Meeting, Orleans, May 2003

Nov 9 :The Movie Mapped onto CGM coordinates

STAMMS Meeting, Orleans, May 2003

Observed distribution of AKR bursts

STAMMS Meeting, Orleans, May 2003

AKR Burst locations

vs.

STAMMS Meeting, Orleans, May 2003

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

STAMMS Meeting, Orleans, May 2003

STAMMS Meeting, Orleans, May 2003

AKR burst mean location drift: example119 Aug 2002,

Southern hemisphere

STAMMS Meeting, Orleans, May 2003

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