Computer Science
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........16z&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF ALABAMA IN HUNTSVILLE, 1993.Source: Dissertation Abstracts International, Volume: 54-12, Section:
Computer Science
Scientific paper
A semi-empirical plasma density model and Mead -Fairfield magnetic field model are incorporated into a 3-D ray tracing code to study magnetospheric ULF wave propagation from the subsolar magnetopause. The ray-tracing of Pc3 compressional waves from the magnetosheath reveals that the magnetosphere can present a major propagation barrier to the penetration of these waves to the plasmasphere. This barrier is the ion-ion cutoff between the He^+ and O ^+ gyroresonances. As a result of the frequency -dependent location of this cutoff, the magnetosphere behaves like a filter for Pc3 compressional waves, and only the low frequency components can penetrate to the inner magnetosphere. These results are in agreement with previous satellite observations. This 'filter action' strongly depends on the relative concentration of He^+ and O^+ and is, therefore, sensitive to solar and magnetic activity. The study of the propagation characteristics of Pc3 transverse Alfven waves shows that these waves cannot penetrate to low Earth altitudes for wave frequencies above about approximately 0.03 hz. The configuration of the refractive index reveals an O^+-He^+ associated cutoff located between the assumed wave source in the equatorial magnetopause and the Earth. When the O^+ concentration is removed from the plasma composition, the barrier no longer exists, and waves with much higher frequencies than 0.03 Hz can penetrate to low altitudes. The result that the 0.03 Hz or lower frequency Alfven waves can be guided to the low altitudes agrees with ground-based power spectrum observations at high latitudes. The ray tracing study of Pc 1-2 waves reproduces earlier results (Rauch and Roux, 1982) for an H ^+-He^+ two-ion-species plasma, i.e. Pc 1-2 left hand polarized Alfven mode waves originating at equatorial geostationary orbit, below He ^+ gyrofrequency, are guided to the ground. However, our ray tracing study shows that previous Pc 1-2 ray tracing results are only valid in the absence of O^+ . In the presence of O^+ concentrations, Pc 1-2 Alfven mode wave are reflected by the pseudo-cutoff introduced by O^+. The ray tracing results also reveal that Pc 1-2 waves generated at the magnetopause or outer magnetospheric regions are organized by a series of 'critical frequencies'. Recent AMPTE CCE observations reported by Anderson et al. (1990) show evidence for the existence of such 'critical frequencies'. The concept of the critical frequency for Pc 1-2 waves can also be well used to explain the observational 'slot' around the He^+ and O^+ gyrofrequencies in the Pc 1-2 wave spectrum at equatorial geosynchronous orbit.
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