Resolute Bay, Nunavut Auroral Radio Emission Sources: Possible Sources of Radar Backscatter? James LaBelle Department of Physics and Astronomy Dartmouth.

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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

3f ce Emission (4-4.5 MHz) 2f ce Emission (near 3 MHz) 02/18/1995 Arviat, Nunavut Universal Time Frequency (kHz) AURORAL MF-BURST AURORAL ROAR EMISSION AURORAL MF-BURST EMISSION

Resonance frequencies vs altitude in the ionosphere

70-79 o invariant, 300 km From: John Hughes’s PhD 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  f uh  2f ce  – s  (1-v /c ) - k v = 0 ce 2 || v +v || T 2  Ellipse in v -v space || T v-parallel [x 10 3 km/s] v-perp [x 10 3 km/s] f uh =2.65 f ce = MHz s= MHz Z-mode Cyclotron maser resonance mechanism s=1,2,3,…

Growth Rate (  /  ce ) log 10 (  /  ce ) (  pe /  ce ) kc/  ce ~ 24  ~ 4 m kc/  ce ~ 8  ~ 12 m Calculations courtesy of P. Yoon Yoon 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 f uh =2f ce OR 3f ce  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

Time After Launch (seconds) f uh  2f ce (expanded view) (expanded view) [from: Samara, LaBelle, Kletzing, and Bounds, 2004]

Auroral Roar 3f ce Emission (4-4.5 MHz) 2f ce Emission (near 3 MHz) 02/18/1995 Arviat, Nunavut Universal Time Frequency (kHz) MF-burst Emission

Auroral Roar Auroral MF Burst Lowest frequency > L-cutoff at F-peak and > f pe E-peak Highest frequency < f pe 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) Electron gyrofrequency Electron plasma Frequency (=3.30 f_ce In this case) Upper Hybrid Frequency (=3.45 f_ce In this case) V beam Langmuir- Z-mode Wavelength >/~ f pe /v beam = 10 m (assuming f pe ~2 MHz, E beam ~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 ~ MHz sources believed to be Langmuir waves? sources believed to be propagating waves wavevector parallel to B, ~10-20 m source altitude km  must look up the field line related to ISR plasma lines or NEIALs?

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