Optical Signatures of Ground-based VLF Transmitter-Induced Electron Precipitation from the Radiation Belts

Details Optical Signatures of Ground-based VLF Transmitter-Induced Electron Precipitation from the Radiation Belts Optical Signatures of Ground-based VLF Transmitter-Induced Electron Precipitation from the Radiation Belts

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsm44a..06m&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #SM44A-06

Physics

[2431] Ionosphere / Ionosphere/Magnetosphere Interactions, [2483] Ionosphere / Wave/Particle Interactions, [2716] Magnetospheric Physics / Energetic Particles: Precipitating, [2774] Magnetospheric Physics / Radiation Belts

Scientific paper

Ground-based VLF transmitters, although designed and built for long-range communication utilizing the efficient wave propagation in the Earth-ionosphere waveguide, nonetheless leak a small fraction of their radiated energy through the ionosphere and into the magnetosphere. The resulting whistler-mode waves propagate either obliquely or along field-aligned density enhancements known as ducts. In the equatorial region, these waves interact with 100—300 keV energetic electrons, which undergo pitch-angle and energy scattering through cyclotron resonance. Electrons near the bounce loss cone may be scattering into the loss cone, thus precipitating in the lower ionosphere at 60—100 km altitude in their subsequent half-bounce period. This precipitation creates significant ionization enhancements, which may be observed by subionospheric VLF probing techniques. Such observations have recently been reported but are still under debate, as direct VLF heating of the ionosphere may contribute to the measured signature. However, the ionization produced by these precipitating electrons will in turn produce optical emissions through excitation of the neutral atmospheric species, and it may be possible that these optical signatures will be observable. Currently experimental efforts are under way to detect this optical signature, which will not be produced by direct VLF heating, so that the measurement will constitute an unambiguous detection of VLF transmitter-induced precipitation. In this paper, we present model calculations that predict an observable optical signature. VLF waves are modeled using the Stanford ray-tracing program, and pitch-angle and energy scattering are calculated through cyclotron resonance. From the resulting precipitating flux, Monte Carlo model is next used to calculate the ionization profiles, as a function of altitude. Using known optical excitation rates, this ionization profile is then turned into optical emissions rates, and photons are propagated to a camera on the ground or a satellite location to find the expected Rayleigh brightness of the emitting region. Results show that the larger ground-based transmitters may produce observable optical emissions.

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