Topliss, Stephen Mark; (2000) Particle features at the equatorward edge of the cusp. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis investigates particle features observed at the equatorward edge of the Earth's magnetospheric cusp regions by instruments onboard NASA's POLAR spacecraft. Two years of data from the TIMAS instrument, a mass-resolved electrostatic ion analyser, are used to identify the location of the cusp. Different types of cusp particle signatures are discussed. The effect of the Interplanetary Magnetic Field (IMF) orientation on cusp location is investigated, and results are found to generally agree with previous, lower-altitude studies. For southward IMF, magnetosheath plasma can enter the magnetosphere through low-latitude reconnection. Examination of HYDRA electron data reveals that only 3% of POLAR cusp crossings have a magnetosheath 'electron-only' region at the equatorward edge of the cusp. Electron spectra within this region suggest that an electric field above POLAR is retarding the electrons in order to maintain charge neutrality. We discuss the validity of deducing the presence of electric fields from single point electron spectrum measurements. Intermittent conic distributions, occasionally peaked close to 90 pitch-angle at the equatorward edge of the cusp, reveal that ionospheric ions are locally accelerated over a 5 to 8 RE altitude range. Evidence of multiple, mass-specific acceleration processes, acting in distinct regions of the cusp is presented. Enhanced wave activity coincides with the period in which intermittent conics are detected. However, no correlation between the appearance of individual conic distributions and peaks in wave power is found. Low-frequency waves of an irregularly pulsed nature are detected at discrete frequencies. Split, or 'butterfly' ion dispersion signatures, detected during periods of northward IMF, have previously been reported and interpreted as evidence of magnetosheath plasma entry by cross-field diffusion. We develop a simple model of reconnection at high latitudes, which shows that these dispersions can be produced on reconnected field lines which accelerate as they move sunward into regions of lower magnetosheath flow velocity.

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