locations of auroral kilometric radiation using a 4-station space-based interferometer
DESCRIPTION
Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer. Robert Mutel, Ivar Christopher University of Iowa. URSI, Boulder, Jan 2006. Image from ISS. Auroral displays. Altitudes of Aurora. (Red). (Green). Aurora: caused by precipitating keV Electrons. - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/1.jpg)
Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer
Robert Mutel, Ivar ChristopherUniversity of Iowa
URSI, Boulder, Jan 2006
![Page 2: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/2.jpg)
Auroral displays
Image from ISS
![Page 3: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/3.jpg)
Altitudes of Aurora
(Red)
(Green)
![Page 4: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/4.jpg)
Aurora: caused by precipitating keV Electrons
![Page 5: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/5.jpg)
Magnetic reconnection: Source of energetic particles for aurora, AKR (?)
![Page 6: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/6.jpg)
Auroral Kilometric RadiationMechanism: Electron Cyclotron Maser Instability
![Page 7: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/7.jpg)
Conditions needed for Cyclotron Maser Instability
•
• Requires fpe/fce << 1
• Requires ∂f/ ∂vperp > 0 e.g. ‘Horseshoe’ velocity distribution
• Terrestrial AKR: occurs on B-field lines in acceleration region above auroral zone
• AKR also detected from all Jovian planets
• May also be responsible for radiation from active stars, AGN Begelman, Ergun, Rees
2005 ApJ
![Page 8: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/8.jpg)
CMI gain, bandwidthCMI Dispersion relation”
For γ<<1, expand:
Growth rate (imaginary part of ω):
![Page 9: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/9.jpg)
![Page 10: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/10.jpg)
AKR Time-frequency spectra
![Page 11: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/11.jpg)
Scientific Question addressed by VLBI location studies of AKR:
What is the detailed correspondence between
AKR bursts (at 2-4 Re) and auroral features (at 80-300
km)?
This addresses the magnetosphere-
Ionosphere coupling problem
![Page 12: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/12.jpg)
AKR location measurements using VLBI (differential delays)
![Page 13: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/13.jpg)
Cluster 4 S/C VLBI array
Requires real-time waveform downlink
(4 DSN antennas!)
![Page 14: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/14.jpg)
Cross-correlate waveforms to measure
differential delays
Note: CCF delay peak width Δτ~ 1/BW
Note: When S/C widely separated, only small fraction of AKR bursts are common to all S/C!
![Page 15: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/15.jpg)
Location-finding algorithm
1. In order to determine the AKR source position, the differential delay is measured between the waveform arrival time for each baseline between pairs of spacecraft. For baseline connecting spacecraft i and j, the differential delay is given by
2. The resulting differential delays are further vetted by examining the signed sum of delays on each of the four independent baseline triangles. For an unresolved source, the signed sum of all delays on baseline triangle (i, j, k) must be zero within the delay uncertainty
3. A cubic grid of 512,000 points is examined, delays calculated for each grid point. All points with delays within uncertainty are cobnsidered valid solutions.
![Page 16: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/16.jpg)
VLBI technical details• Snapshot mode! (bursts lasts only ~1s -10s)
•Timing uncertainty ±10 μs << 1ms delay uncertainty
• Clock: ‘ultra’ stable crystal: δt ~ 10-10 s/s
• Real-time downlink (waveforms, 8-bit)
• Time: DSN stamped ±1 μs
• Essentially no location determination along z (l.o.s.)
• 6 baselines: over-determined position solution (3 unknowns: X,Y,Z)
• Source unresolved (coherent emission, high Tb)
• New VLBI maps every 300ms x 12 freq chan
• Frequency channels 12 x 1 KHz
• Only delays used – not phases (yet)
• S/C positions uncertainty ~1km (3 μs error)
![Page 17: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/17.jpg)
Position Uncertainty
If we displace a source in the plane perpendicular to the line of sight by distance x along the direction of B, the corresponding differential delay will be
Assuming x << B << z, we can easily solve for x
~ 500 -1,000 km
Assuming x << B << z, a calculation similar to the above, but for a source offset by B/2 in the perpendicular plane results in a position uncertainty
~ 5,000 - 20,000 km
![Page 18: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/18.jpg)
AKR VLBI science results
1. Association of AKR burst locations with discrete auroral arcs, but:
1. Some (up to 50%) AKR bursts have no associated auroral features
2. Some discrete arcs have no AKR emission
2. Motion of AKR burst centroids (V ~ 100 m/s)
3. Radiated beamwidth of AKR bursts are much smaller than previously reported (HPBW~ 5°)
4. Striated AKR triggered by ion stationary structures
![Page 19: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/19.jpg)
Time variations in AKR burst locations
9 Nov 2003 (3 hours)
(125, 250, 500 KHz)
29 Dec 2002 (1 ½ hrs)
![Page 20: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/20.jpg)
Simultaneous AKR location/UV imaging of aurora
![Page 21: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/21.jpg)
Correlating Narrowband (Striated) AKR bursts• Problem: Narrow bandwidth implies large uncertainty in
delay and location:– Δν ~ 50 Hz → Δτ ~ 20 ms → Δx ~ 4 Re!
• Solution: Cross-correlate using longer time window, isolated bursts
• Example below: cross–correlation of Synthetic Monochromatic Chirp Signals (τ = 10 ms, Signal + UDP phasor)
Δτ ≈ 0.5 ms
![Page 22: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/22.jpg)
Summary1. VLBI Technical Summary
1. 4 S/C VLBI successfully done from space (no ground stations)
2. Real-time data downlink to DSN (37 kbps) – time stamps added
3. Software Correlator (IDL)
4. Onboard crystal Local oscillator, stability: 10-10 s/s (100s)
5. Uses differential delays from CCF peak (no phase)
6. Position determination only - no source structure (unresolved)
7. Position uncertainties ~ ± 200km - 400km at Earth
8. Narrowband AKR drifting bursts can be mapped using chirp analysis
2. AKR Science
1. AKR emission occurs on B field lines associated with discrete auroral arcs
2. AKR activity center drifts at 80-100 m/s over hours
3. AKR beam sizes are much smaller than previously assumed (~5°)
4. Striated AKR triggered by ion stationary structures
![Page 23: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/23.jpg)
![Page 24: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/24.jpg)
![Page 25: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/25.jpg)
![Page 26: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/26.jpg)
![Page 27: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/27.jpg)
![Page 28: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/28.jpg)
![Page 29: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/29.jpg)
![Page 30: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/30.jpg)
![Page 31: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/31.jpg)
![Page 32: Locations of Auroral Kilometric Radiation Using a 4-station Space-Based Interferometer](https://reader034.vdocument.in/reader034/viewer/2022051418/5681585f550346895dc5ba3b/html5/thumbnails/32.jpg)