An investigation of cusp latitude magnetosphere-ionosphere physics: A time series analysis approach

Abstract

The shocked solar wind plasma of the magnetosheath has direct access to the Earth's high-latitude ionosphere and upper atmosphere only through the magnetospheric cusps. The interaction of solar and terrestrial plasmas and fields in these regions has made them an obvious choice for the study of coupling processes in the geospace environment. Some of the information regarding these processes is manifest in the transmission and generation of wave energy, a portion of which can be detected by ground-based magnetometers. In the present day, records of the magnetic field are stored in a digital format; therefore, some form of signal processing is required to extract meaningful physical information from them. This thesis is aimed at the physical characterization of the cusp region through the careful application of digital time series analysis techniques to ground-based magnetometer records. It is demonstrated that judicious application of signal processing techniques can yield new, physically meaningful results from ground-based magnetometer records, and aid in the understanding of disparate reports from groups using different analysis techniques on like data. Characterization of the cusp region is couched in terms of three specific, open problems of the physics of magnetic perturbations in the cusp: (1) the coherence of localized pulsations, (2) the spatiotemporal nature of the cusp magnetic spectrum, and (3) the ground-based magnetic determination of the separatrix. The first problem is addressed by assuming that localized pulsations are coherent only over some finite spatial extent. A statistical measure of interstation coherence is developed to estimate an upper bound of ${\cal O}$(200 km) for the coherence length of this class of pulsations. The second problem is addressed by examining the ultra low frequency polarization spectrum. An information theoretic measure is established as a quantitative means of discriminating the spatial passage of the cusp by ground-based magnetic means. This procedure replaces previous determinations which were made "by-eye." Finally, separatrix identification is addressed by applying the statistical interstation coherence measure to pulsations presumably representative of a magnetic field line resonance. The analysis indicates that a determination is not possible to a resolution better than ${\cal O}$3(300 km).