Computer Science
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........30b&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF CALIFORNIA, LOS ANGELES, 1995.Source: Dissertation Abstracts International, Volume: 56-01, Sectio
Computer Science
Scientific paper
Electromagnetic waves in the audio frequency range, propagating in the regions of plasma surrounding the Earth, have quite different properties than their counterparts in vacuum and are known as whistler waves. These waves are observed commonly to propagate in the plasmasphere, auroral regions, and ionosphere, as well as in more distant space plasmas including the solar wind and various planetary magnetospheres. The behavior of whistler waves in uniform plasmas or plasmas with gentle density gradients has been studied theoretically and in laboratory experiments and is well understood. The ionosphere, however, is characterized by the presence of field-aligned density striations. These striations are very long, narrow regions where the plasma density is depleted compared to the background. At the boundaries of striations are found short scale-length density gradients. So the problem of interest is to understand how whistler waves interact with these striations. Some investigators predict that whistler waves should undergo linear mode-coupling, giving rise to different, shorter wavelength waves (referred to as lower hybrid waves) in the presence of these gradients and, indeed, recent satellite data yield evidence in this direction. In the laboratory, the behavior of whistler waves is investigated in a plasma containing a single density striation. The electromagnetic fields of a launched whistler wave are measured in the vicinity of the striation for a wide range of plasma parameters. The parameters are varied systematically so that the experiments are a function of the ratio of whistler wavelength to striation diameter. Information is yielded by the data about wavelengths that are present and it is seen that lower hybrid waves are created with wavelengths consistent with the linear mode-coupling model. The amplitude of these waves, although smaller than the incident waves', is nonetheless significant. Spatially, the interaction is concentrated in the striation gradient region upon which the whistler waves impinge. An unusual amplitude minimum region appears as well as two unexpected wave modes. Measurements are also made in a near-uniform plasma for comparison, both to previous work and to predictions of a Green's function analysis for a cold, infinite plasma.
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