Magnetospheric Pulsations: Models and Observations of Compressional Waves

The first part of the dissertation models ultralow frequency (ULF) waves in a simplified geometry in order to understand the physics of the mode coupling between the compressional and shear Alfven waves in an inhomogeneous magnetized plasma. Wave mode coupling occurs when a field line resonant frequency (defined by the shear Alfven mode) matches the global mode frequency (defined by the compressional mode). Large wave amplitudes occur near the resonant field line. Although the wave amplitude of the global mode is small away from resonant field lines, significant wave energy is stored in the wave mode due to its large scale nature. It serves as a reservoir to continuously feed energy to resonant field lines. This mechanism may explain why some field line resonances can last for times longer than that predicted from the ionospheric Joule dissipation. A nonmonotonic Alfven velocity divides the magnetosphere into two or more cavities by the local maxima of the Alfven velocity. The global mode is typically localized in one of the cavities except at some preferred frequencies, the global mode can extend through more than one cavity. This may explain ULF wave excitations in the low latitude magnetosphere. The second part of the dissertation is devoted to study compressional waves in the outer magnetosphere using magnetic field and plasma data. Statistical information on the distribution of compressional Pc 5 waves in the outer magnetosphere is obtained. Large amplitude, long period compressional Pc 5 pulsations are found very common near the magnetic equator. They are polarized mainly in a meridian plane with comparable compressional and transverse amplitudes. Close correlation between compressional wave amplitude and plasma beta is also found. Several case studies show that compressional waves are quenched in the region where beta < 1. In the outer magnetosphere, the criterion for the mirror instability is found to be frequently satisfied near the magnetic equator if large amplitude compressional waves are present. One detailed event study allows us to determine the transverse wavelengths in the radial and the azimuthal directions. The observed properties of the waves are used to constrain the possible wave generation mechanisms.