Resolute Bay, Nunavut

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Resolute Bay, Nunavut
Auroral Radio Emission Sources Possible Sources
of Radar Backscatter?
James LaBelle Department of Physics and
Astronomy Dartmouth College SuperDARN
workshop Hanover, New Hampshire May 31, 2011
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AURORAL ROAR EMISSION
3fce Emission (4-4.5 MHz)
2fce Emission (near 3 MHz)
AURORAL MF-BURST
AURORAL MF-BURST EMISSION
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Resonance frequencies vs altitude in the
ionosphere
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70-79o invariant, 300 km
From John Hughess PhD Thesis (2000)
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At right Measured electron distribution
function, with Z-mode cyclotron maser resonance
condition for two selected frequencies superposed
(courtesy S.R. Bounds and C.A. Kletzing,
University of Iowa)Condition for spontaneous
emission f _at_ fuh _at_ 2fce
Z-mode Cyclotron maser resonance
mechanism

• w sw (1-v /c ) - k v 0

2 2
? Ellipse in v -v space

ce

T
s1,2,3,
v-parallel x 103 km/s -50
0 50
fuh2.65 fce1.35 2.65 MHz s2
s2 2.66 MHz
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Growth Rate (g/wce)
Wavenumber (kc/wce)
kc/wce 24 ? l 4 m
kc/wce 8 ? l 12 m
Wavelength is correct for superDARN, but is the
backscatter freq offset by 3 MHz?
log10(g/wce)
Calculations courtesy of P. Yoon Yoon et al., J.
Geophys. Res., 103(A3), 4071, 1998
(wpe/wce)
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Resonance frequencies vs altitude in the
ionosphere
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(No Transcript)
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Churchill, Manitoba
from Shepherd et al., 1999
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1. AURORAL ELECTRONS UPPER HYBRID
fuh2fce OR 3fce ? DISCRETE
EIGENMODES Z-MODE CYCL MASER DUE TO
INHOMOGENEITY
(mode conversion)
2. MODE CONVERSION ? STRUCTURED
O-MODE WAVES ESCAPE RADIO
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Radar backscatter
from Yoon, Weatherwax, LaBelle, 2000
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fuh_at_2fce
272 274 276 278
280 282 284 Time After Launch
(seconds)
(expanded view)
(expanded view)
from Samara, LaBelle, Kletzing, and Bounds,
2004
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Auroral Roar 3fce Emission (4-4.5 MHz)
2fce Emission (near 3 MHz)
MF-burst Emission
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Auroral Roar Auroral MF
Burst
Relatively low energy electron beam ( hundreds
of eV)
Lowest frequency gt L-cutoff at F-peak
and gt fpe E-peak
Highest frequency lt fpe F-peak
MFB on poleward edge of expanding substorm
aurora Requires low energy ? consistent with
Alfvenic electron beam wave
aurora at poleward edge
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Vbeam
Electron plasma Frequency (3.30 f_ce In this
case)
Upper Hybrid Frequency (3.45 f_ce In this case)
Langmuir- Z-mode
Electron gyrofrequency
Wavelength l gt/ fpe/vbeam 10 m (assuming
fpe2 MHz, Ebeam1 keV)
Dispersion Surfaces (from Andre, 1985)
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Auroral roar emissions Narrowband, 3, 4.5
MHz
sources are upper hybrid waves
sources believed to be
standing waves
wavevector perp to B, l 4-12 m
source altitude
near 300 km ? good candidate for
superDARN scatter? frequency
offset! Auroral MF burst emissions Broadband
1.5-4.5 MHz
sources believed to be Langmuir waves?
sources believed to be
propagating waves
wavevector parallel to B, l10-20 m
source altitude 300-500 km
? must look up the field line
related to ISR plasma lines or
NEIALs?
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