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Auroral Emission Sources: Radar Backscatter Origins?

Investigating potential sources of radar backscatter in Resolute Bay, Nunavut. Detailed examination of emission frequencies, resonance mechanisms, electron distributions, and growth rates in the ionosphere. Analysis conducted based on findings from James LaBelle’s research and John Hughes’s PhD Thesis, focusing on Z-mode Cyclotron maser resonance and spontaneous emissions. Calculations provided by P. Yoon and colleagues. Exploration of auroral electrons, upper hybrid frequencies, and structured O-mode waves, with considerations for mode conversion effects. Examination of narrowband and broadband auroral roar and MF-burst emissions, their potential sources, altitude, and wave characteristics. Querying the offset frequencies for backscatter in relation to SuperDARN observations.

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Auroral Emission Sources: Radar Backscatter Origins?

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  1. 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

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

  3. Resonance frequencies vs altitude in the ionosphere

  4. 70-79o invariant, 300 km From: John Hughes’s PhD Thesis (2000)

  5. Z-mode Cyclotron maser resonance mechanism 2 2 v +v || T 50 0 v-perp [x 103 km/s] -50 w – sw (1-v /c ) - k v = 0 2 2 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  Ellipse in v -v space || || ce || T s=1,2,3,… v-parallel [x 103 km/s] -50 0 50 fuh=2.65 fce=1.35 2.65 MHz s=2 s=2 2.66 MHz

  6. 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)

  7. Resonance frequencies vs altitude in the ionosphere

  8. Churchill, Manitoba [from: Shepherd et al., 1999]

  9. 1. AURORAL ELECTRONS UPPER HYBRID fuh=2fce OR 3fce  DISCRETE EIGENMODES Z-MODE CYCL MASER DUE TO INHOMOGENEITY (mode conversion) 2. MODE CONVERSION  STRUCTURED O-MODE WAVES ESCAPE RADIO

  10. Radar backscatter [from: Yoon, Weatherwax, LaBelle, 2000]

  11. fuh@2fce 272 274 276 278 280 282 284 Time After Launch (seconds) (expanded view) (expanded view) [from: Samara, LaBelle, Kletzing, and Bounds, 2004]

  12. 02/18/1995 Arviat, Nunavut Frequency (kHz) Universal Time Auroral Roar 3fce Emission (4-4.5 MHz) 2fce Emission (near 3 MHz) MF-burst Emission

  13. Auroral Roar Auroral MF Burst Relatively low energy electron beam (~ hundreds of eV) Lowest frequency > L-cutoff at F-peak and > fpe E-peak Highest frequency < fpe F-peak MFB on poleward edge of expanding substorm aurora Requires low energy  consistent with Alfvenic electron beam “wave aurora” at poleward edge

  14. 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 >/~ fpe/vbeam = 10 m (assuming fpe~2 MHz, Ebeam~1 keV) Dispersion Surfaces (from Andre, 1985)

  15. 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, l~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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