resolute bay, nunavut auroral radio emission sources: possible sources of radar backscatter? james...
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![Page 1: Resolute Bay, Nunavut Auroral Radio Emission Sources: Possible Sources of Radar Backscatter? James LaBelle Department of Physics and Astronomy Dartmouth](https://reader038.vdocuments.us/reader038/viewer/2022110207/56649d475503460f94a22b2e/html5/thumbnails/1.jpg)
Resolute Bay, NunavutAuroral Radio Emission Sources: Possible Sources of Radar Backscatter?
James LaBelleDepartment of Physics and AstronomyDartmouth College
SuperDARN workshopHanover, New HampshireMay 31, 2011
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3fce Emission (4-4.5 MHz)
2fce Emission (near 3 MHz)
02/18/1995 Arviat, Nunavut
Universal Time
Fre
qu
ency
(k
Hz)
AURORAL MF-BURST
AURORAL ROAR EMISSION
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 Hughes’sPhD 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 fuh 2fce
– s (1-v /c ) - k v = 0ce
2 2|| ||
v +v ||
T2 2 Ellipse in v -v space||
T
v-parallel [x 103 km/s]-50 0 50
v-p
erp
[x 1
03 km
/s]
50
-50
0
fuh=2.65 fce=1.352.65 MHzs=2
s=22.66 MHz
Z-mode Cyclotron maser resonance mechanism
s=1,2,3,…
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Gro
wth
Rat
e (
/ce
)
log 10
(/
ce)
(pe/ce)
kc/ce ~ 24 ~ 4 mkc/ce ~ 8 ~ 12 m
Calculations courtesy of P. YoonYoon et al., J. Geophys. Res., 103(A3), 4071, 1998
Wavelength is correct for superDARN, but: is the backscatter freq
offset by ~3 MHz?
Wavenumber (kc/ce)
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Resonance frequencies vs altitude in the ionosphere
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[from: Shepherd et al., 1999]
Churchill, Manitoba
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(mode conversion)
1. AURORAL ELECTRONS UPPER HYBRID fuh=2fce OR 3fce DISCRETE EIGENMODES Z-MODE CYCL MASER DUE TO INHOMOGENEITY
2. MODE CONVERSION STRUCTURED O-MODE WAVES ESCAPE RADIO
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[from: Yoon, Weatherwax, LaBelle, 2000]
Radar backscatter
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272 274 276 278 280 282 284Time After Launch (seconds)
fuh2fce
(expanded view)
(expanded view)
[from: Samara, LaBelle, Kletzing, and Bounds, 2004]
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Auroral Roar3fce Emission (4-4.5 MHz)
2fce Emission (near 3 MHz)
02/18/1995 Arviat, Nunavut
Universal Time
Fre
qu
ency
(k
Hz)
MF-burst Emission
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Auroral Roar Auroral MF Burst
Lowest frequency > L-cutoff at F-peak and > fpe E-peak
Highest frequency < fpe F-peak
Relatively low energy electron beam (~ hundreds of eV)
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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Dispersion Surfaces(from Andre, 1985)
Electrongyrofrequency
Electron plasmaFrequency(=3.30 f_ceIn this case)
Upper Hybrid Frequency(=3.45 f_ce
In this case)
Vbeam
Langmuir-Z-mode
Wavelength >/~ fpe/vbeam = 10 m(assuming fpe~2 MHz, Ebeam~1 keV)
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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, ~ 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, ~10-20 m source altitude 300-500 km
must look up the field line related to ISR plasma lines or NEIALs?
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`