20 September 2005 Double Star -Cluster Noordwijk Sept 2005 1
WBD Studies of AKR: Coordinated
Observations of Aurora and the SAKR – Ion Hole Connection
R. L. Mutel
University of Iowa
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Outline• Summary of AKR Properties • AKR-Auroral Studies
– VLBI technique, uncertainties– AKR – Auroral zone source location studies – Example: 8 June 2004
• Striated AKR SAKR – Ion ‘holes’– Summary of SAKR properties– Comparison with Ion Solitary Structures– Do Ion solitary structures trigger SAKR?
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Characteristics of Terrestrial Auroral Kilometric Radiation
• Detected 25% - 50% of time
•Seasonal dependence: more frequent in winter
• Strongly associated w. geomagnetic activity, aurora
• Frequency 50 KHz – 1 MHz
• Satellite detection only (ionosphere blocks terrestrial reception)
• Generation mechanism: e- cyclotron maser
• Requires e- distribution function:
• Freq-time spectra show complex structure
• Broadband drifting bursts last ~hrs
• Fine structure – narrowband, drifting timescale ~ seconds
• SAKR (Striated AKR)
• Negative freq. drift => V~100-200 km/s
• Assoc w. solitary structures? (focus of this talk)
0f
V
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AKR Source region:
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
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FAST Observations of AKR Source Region (Ergun et al. Ap. J. 538, 456)
Shell instabilityShell instability
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Closed cycle system
connecting AKR with auroral
region
From Strangeway et al. 2000
• Free energy source for AKR consists of accelerated primary electrons (downward) and mirrored electrons (upward) with substantial V┴
• Mirroring (& turbulence) scatters beamed (V||)electrons in velocity space
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Sample AKR dynamical spectra showing effect of angular beaming (see Christopher et al. poster Thursday)
SC1-SC4 angular sep’n ~14º
SC1-SC4 angular sep’n ~30º
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VLBI Source Location Algorithm: Differential delay measurement
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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
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Fly-through of AKR positions
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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:x
Typical uncertainty in plane:
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Movies of temporal behavior of Aurora, AKR
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AKR Source
Electron acceleration
region
Auroral bright spot
Case 1: Electron Acceleration Region Above AKR Source Region
Nominal Predictions:
• High Correlation Between AKR Source and Auroral Bright Features on Same Magnetic Field line
• Assumes electron precipitation continues to auroral region [suitable f(v)]
• But upward mirrored elecgtrons can still contribute
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AKR Source
Electron acceleration
region
Auroral bright spot
Case 2: Electron Acceleration Region Below AKR Source Region
Nominal Predictions:
• Low Correlation Between AKR Source and Auroral Bright Features on Same Magnetic Field line
• If AKR emission, sources not on same field line
• Possibility that upward scattered electrons could trigger AKR
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IMAGE Auroral Images 8 June 2004 11:30 – 11:49 UT
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AKR Source Locations8 June 2004 (11:13 – 11:51 UT)
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Both movies
Striated AKR (SAKR)
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Small Angular Sizes imply much lower ERS luminosities (10-100 W) vs. ~104 W previously assumed)
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Summary of SAKR Properties and Implied properties of trigger
Observed SAKR Property
Implied Trigger Property
Negative slope Upward traveling
Slope -3 → -12 km/s Speed 75 – 300 km/s
Narrowband (20 - 40 Hz)
Spatial scale < 1 km (along B field)
Much more common at low frequency (100 KHz vs. 500 KHz
Trigger more common at higher altitude
(R > 2.5Re)
High intensity (comparable with normal AKR
E field must disturb electron f(v) significantly (for max. growth)
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Ion and Electron Solitary Structures (Bounds et al. 1999 JGR)
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Ion solitary structures move up B field at V ~75 - 300 km/s (Bounds et al. 1999)
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Dipolar and Tripolar structures seen by Cluster 4.5 -6.5 Re (Pickett et al. 2004)
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Velocity of tri-polar structure from Cluster SC lags
V~ 925 km/s
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Ion hole and tri-polar solitary structures: Consistent with narrow bandwidths
10t ms
1100 300
3
ia
z
V km s
L km
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Bandwidths in excellent agreement
4
4
3 2 10
:
0.021.6 10
125
B
B
B
B
B r
B r
Observed
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Ion holes: trigger for
striated AKR
bursts?
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Electron-Cyclotron Maser (ECM) Resonance condition
12 22
2 2
2 2
2
0
1, 1 1
2
1 02
ce
cece
ce
k v n
v v
c c
v vk v n
c
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Electron energy dist function (‘horseshoe’) in magnetospheric acceleration region
(Polar in situ obs)
0f
V
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Observed SAKR Property
Implied Trigger Property
Ion Solitary Structures
Negative slope Upward traveling Yes
Slope -3 → -12 km/s Speed 75 – 300 km/s Yes (e.g. Bounds, 1999)
Narrowband (20 - 40 Hz)
Spatial scale < 1 km (along B field)
Yes (many refs)
Much more common at low frequency (100 KHz vs. 500 KHz
Trigger more common at higher altitude
(R > 2.5Re)
Unknown (but FAST measures in situ)
High intensity (comparable with normal AKR
E field must disturb electron f(v) significantly (for max. growth)
dE/dx ~ 0.5 V/m, Vtot ~ 0.1 kV. Requires detailed PIC code to verify
SAKR: A Tracer of Ion Solitary Structures?
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Summary
• Preliminary joint AKR (Cluster WBD VLBI) imaging with aurora (IMAGE) shows some correlation (but not high)– Is electron current return inefficient?– Ongoing joint observations started June 05 should
clarify connection
• Striated AKR may be triggered by ion ‘holes’ (solitary structures)– Most observed parameters consistent with ion holes– Detailed PIC calculation, analytic integration of growth
rate needed for verify
Recommended