An investigation of Ultra Low Frequency (ULF) pulsations was conducted using data

Micropulsation data recorded at Palo Alto, California during 1963-4 and Ralston, Alberta during 1967 have been used to study "Intervals of pulsations of diminishing period" (IPDP). IPDP's are found to be generated in the dusk-midnight quadrant of the magnetosphere at an equatorial distance of about 6 earth radii. An intensive study of the Ralston data reveals that IPDP's occur during the expansive phase of magnetospheric substorms. It is proposed that IPDP's are generated by a cyclotron instability between energetic protons and left-hand ion cyclotron waves. Their main characteristics are determined by the perturbations of the dusk-midnight sector of the magnetosphere by magnetospheric substorms. One of the main disturbances in that region is a slow decrease and then sudden increase in the magnetic field corresponding to the buildup and decay of a partial ring current. IPDP's show an increase in midfrequency due to the change in the cyclotron instability frequency produced by the increasing magnetic field. This theory is tested by a comparison of frequency increase of IPDP's observed at Ralston and magnetic field increase in the magnetosphere observed by the ATS-1 satellite. Other conditions necessary for IPDP generation are then discussed. It is shown that different combinations of these conditions result in the generation of hm emissions and band type micropulsations. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

A magnetohydrodynamic (MHD) cavity-resonance model is developed to study the ultra low frequency (ULF) response in the magnetosphere to an external compressional impulse. It is assumed that the magnitude of the impulse is small enough such that non-linear terms remain negligible. The MHD differential equations are derived in a cold, non-uniform plasma imbedded in a cylindrical ambient field geometry and are solved using numerical finite difference integration methods. The crucial feature of the model is that it allows for the investigation of the response within the magnetospheric cavity to an impulse that has both temporal and spatial form. There is strong observational evidence that low-latitude Pi 2 pulsations have, or are associated with, a global propagation mechanism. Evidence alluding to the global nature of low-latitude Pi 2 is the characteristically low azimuthal (or axial) wavenumbers, (Irnl ;S 1 ). Further evidence of the global nature of Pi 2 is the lack of arrival time difference between globally separate events, as well as the similarity in the spectral content of globally separate events. As an application, the cavity-resonance model is applied to investigate the Pi 2 pulsation event. The cavity-resonance waves are excited by an impulsive perturbation at the magnetopause which is centred about the midnight meridian. The excitation signal is chosen representing the causal Pi 2 mechanism thought to be associated with the sudden, short circuiting of the cross-tail current to the auroral oval. Various aspects of the cavity-resonance wave modes are investigated and the appropriateness of this type of modelling for -the study of Pi 2 is evaluated. Numerical integration and well as Fourier and Laplace methods are used to investigate the transmission of the impulsive signal through the magnetosphere. Coupling between the isotropic (cavity) and the transverse Alfven (resonance) mode is studied. The effect of the plasmapause is considered. Longitudinal variations of polarization as well as the latitudinal phase variations of the perturbed fields are computed. Computational results are compared with observational features of the Pi 2 event. / Thesis (Ph.D.)-University of Natal, Durban, 1993.