The Spectral Characteristics and the Propagation in the Magnetosphere of Hydromagnetic Emissions (Pearls)

摘要

Many of the observed characteristics of hydromagnetic emissions or pearls have been successfully explained by the propagation of the guided Alfven wave along geomagnetic lines of force. This thesis extends some of the previous work, which has concentrated on the dispersion properties of these signals, by examining the fine structure in the spectral character of these signals. These emissions all show a very dramatic spectral feature of a series of high Q peaks. This frequency structure is present in the individual dispersed wave trains, and should not be confused with the fine structure obtained in a Fourier analysis of a series of repetitive signals. It is shown that the spectral peaks that are observed are consistent with the high order field line resonances that would be expected from models which satisfy the observed dispersion. The mode numbers are so high, being of the order 20 to 50, that one cannot expect to predict the actual frequencies observed. The observed separations between the peaks are in good agreement with those predicted from the model studies. An interpretation is proposed that the emissions are produced by the cyclotron resonance amplification of a background wave spectrum of resonant field line oscillations. (Author)