Resolute Bay, Nunavut

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?

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