Digital Spectra of Artificially Stimulated VLF Emissions.
STANFORD UNIV CALIF STANFORD ELECTRONICS LABS
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New features of artificially stimulated VLF emissions from the magnetosphere have been discovered by using high resolution digital spectral analysis techniques. Several hundred emissions triggered by three VLF transmitters NAA, Omega, Siple Station, Antarctica have been examined. The stimulated emissions initially grow at the frequency of the triggering signal. The emissions always initially rise above the frequency of the triggering signal this rise is independent of the final slope of the emissions. The emissions characteristically show exponential growth in time with rates that vary from 25 to 250 dBs. Growth may continue past termination of the triggering signal and appears to be independent of the emission frequency over a range of at least several hundred Hz. The growth also may cease prior to termination. Emission amplitudes may reach 30 dB or more above the level of the triggering signal. Studies of a limited set of the data indicate that lower growth rates are observed when emission activity is just beginning or just ending. These periods are usually characterized by falling rather than rising emissions. These observations are in good agreement with a theory that attributes the emissions to an interaction between the triggering signal and counter-streaming gyroresonant electrons.
- Atmospheric Physics
- Radiofrequency Wave Propagation