Landau damping of magnetospherically reflected whistlers
- Univ. of California, Los Angeles, CA (United States)
- Natural Environment Research Council, Cambridge (United Kingdom)
Unducted VLF signals produced by lightning activity can form a population of magnetospherically reflected (MR) whistlers in the inner magnetosphere. It has been suggested recently that in the absence of significant attenuation such waves could merge into a broadband continuum with sufficient intensity to account for plasmaspheric hiss. To test this conjecture the authors have evaluated the path-integrated attenuation of MR whistlers along representative ray paths using the HOTRAY code. Using a realistic plasma distribution modeled on in situ data, they find that the majority of MR waves experience significant damping after a few transits across the equator. This is primarily due to Landau resonance with suprathermal (0.1-1 keV) electrons. Selected waves that originate at intermediate latitudes (15{degrees} {le} {lambda} {le} 35{degrees}) and whose ray paths are confined to the inner plasmasphere may experience up to 10 magnetospheric reflections before substantial attenuation occurs. These waves should form the population of observed MR waves. Wave attenuation becomes more pronounced at higher frequencies; this can account for the absence of multiple-hop waves above 5 kHz. Weakly attenuated MR waves tend to migrate outward to the L shell, where their frequency is comparable to the equatorial lower hybrid frequency. The enhanced concentration of waves due to a merging of ray paths would produce a spectral feature that rises in frequency at lower L. This is quite distinct from the reported properties of plasmaspheric hiss, which maintains a constant frequency band throughout the entire plasmasphere. Futhermore, in the absence of mode conversion, waves below 500 Hz, which often form an important if not dominant part of the spectral properties of hiss, are unable to escape from the topside ionosphere in the whistler mode. Consequently, the authors conclude that unducted lightning signals cannot account for the origin of plasmaspheric hiss. 9 refs., 13 figs., 1 tab.
- OSTI ID:
- 166256
- Journal Information:
- Journal of Geophysical Research, Vol. 99, Issue A9; Other Information: PBD: 1 Sep 1994
- Country of Publication:
- United States
- Language:
- English
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