1 toward predicting vlf triggering muri workshop 3 march 2008 e. mishin and a. gibby boston college...
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TOWARD PREDICTING VLF TRIGGERING
MURI Workshop
3 March 2008
E. Mishin and A. Gibby Boston College ISR
Stanford University STAR Lab
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OUTLINE
Experimental constraints
Significance of plasmaspheric hiss: Step-like electron distribution
Results of ongoing study
OBJECTIVE: Specify VLF triggering conditions
APPROACH
Compare the occurrence of VLF triggering from the Siple transmitter with the magnetic activity and natural VLF emissions
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Perturbed plasmasphere contains density irregularities
Substorms/storms inject ~10-keV electrons into the plasmasphere
Energetic electrons remain trapped inside the PP for many hours
Necessary conditions for VLF triggering:
Background (unstable) population of >10-keV electrons
Field-aligned plasmaspheric density enhancements (ducts)
substorm aftermath
too vague condition to be useful for predictions
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Hiss Chorus: Natural triggering events (BroadBandE DiscreteE)
R: Rising chorus emission growing from the top of the hiss band
GEOTAIL [Nunn et al., 1997]R
F
85 dB/s
Nunn et al.’ simulation results
The distribution function observed in situ
yields only 10 dB/s Theory failure? DF and amplification must be considered in detail
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Step-like DF: Significance of plasmaspheric hissafter Trakhtengerts + [ 1985-2003]
The “temperature” anisotropy
Eq. pitch-angle
energy
The maximum frequency of hiss
moderately-strong hiss wave-particle interaction leads to diffusion over pitch-angles and precipitation of resonant
particles. In a steady-state, a sharp
increase (step) forms near the separatrix.
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Amplification of a ducted VLF wave
smooth = (2-6) • 0
Broad frequency range
dB
linear growth rate
dB
Step-like EDF
(ka)1/3 = 10-20
A wavelet on the top of the hiss band, f /fmax=1+ (ka)-2/3, gains most
Loss-cone“temperature” anisotropy
[dB] = 4.3•
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Triggering from the Siple VLF transmitter
Database:
June 1986 campaign (at dawn)
Auroral and RC indices
“Operator’s records”
APPROACH
Compare the occurrence of VLF triggering from Siple with (1) previous magnetic activity and (2) the current background VLF noise
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Triggering from Siple (cont’d)
+ fSiple * fav hiss
chorus pow.line
. whistler o nat. trigg. x sferics
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SUMMARY
•Most favorable conditions for VLF triggering in the morning sector seem to be satisfied after weak/moderate substorms.
•The triggering occurred when broad-band hiss emissions were present and the pump frequency was above the top of the hiss band.
•Consistent with theory, which is based on the second-order resonance and accounts for the step-like background electron distribution formed due to interaction with broad-band hiss.
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Formation of a step-like DF
is the Heaviside step function I ┴ = m· is the 1st adiabatic invariant
W = m·= is the kinetic energy
bounce period
diffusion time
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Schematic of VLF triggering
•Propagating toward the equator, ducted pump signals gain energy from background energetic electrons, being amplified by 20-40 dB.
•Near the magnetic equator, phase-trapping of resonant electrons by the amplified pump wave results in the formation of a phase-coherent, quasi-monoenergetic electron beam.
•The beam generates narrow-band VLF emissions with falling or rising frequency. Their sweep rates (~1 kHz/s) satisfy the (generalized) second-order resonance (beam-wave phase coherence ).
SIPLE
receiver
notably, Helliwell +STAR team, Sudan , Nunn, Karpman+ , Omura, Matsumoto, Trakhtengerts+
L=4.2