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. 02/18/1995 Arviat, Nunavut. - PowerPoint PPT PresentationTRANSCRIPT
Resolute Bay, NunavutAuroral Radio Emission Sources: Possible Sources of Radar Backscatter?
James LaBelleDepartment of Physics and AstronomyDartmouth College
SuperDARN workshopHanover, New HampshireMay 31, 2011
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
Resonance frequencies vs altitude in the ionosphere
70-79o invariant, 300 km
From: John Hughes’sPhD Thesis (2000)
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,…
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)
Resonance frequencies vs altitude in the ionosphere
[from: Shepherd et al., 1999]
Churchill, Manitoba
(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
[from: Yoon, Weatherwax, LaBelle, 2000]
Radar backscatter
272 274 276 278 280 282 284Time After Launch (seconds)
fuh2fce
(expanded view)
(expanded view)
[from: Samara, LaBelle, Kletzing, and Bounds, 2004]
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
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
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)
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?
`