Determination and analysis of the sub-second fluctuations in the ionospheric electron density using the GPS
University of New Brunswick
Structures in the high latitude ionospheric density are responsible for variations in the phase and amplitude of trans-ionospheric signals. These include space-based navigation systems like the Global Positioning System (GPS). The ionosphere is dispersive, and the refractive index is dependent on the electron density. Thus, variations in the ionosphere will cause changes in the refractive index of the medium. This will lead directly to fluctuations in the phase of the signal, as observed at ground-based receivers. If the spatial scale of these changes in the electron density is sufficiently small, diffraction can occur. This causes rapid variations in the phase and amplitude of the signal. The study of these refractive and diffractive variations (scintillation) has proven vital in monitoring and studying the ionospheric medium. It is known that the diffractive variations in the signal’s amplitude and phase are observed in the high-frequency regime; in the typical modern analysis, the refractive variations are assumed to be of low frequency and thus removed by high pass filtering the received signal. This assumption is based on longstanding low latitude measurements. In the high latitude, these assumptions must be reevaluated because of the increase in ionospheric drift speeds observed in the region. Without careful evaluation, the high-frequency refractive variations can be wrongly classified as diffraction. On the other hand, in studying high-frequency variations in the electron density, diffractive artifacts may appear as refractive. This study verifies the existence of diffractive-like high-frequency refractive variations in the high latitude and outlines methods to determine whether the high-frequency variations observed in the GPS carrier phase observable are purely refractive. These methods rely on recent advances in the GPS satellite and receivers; the advances and their impact on ionospheric monitoring are discussed. The outlined methods of distinguishing these refractive variations are used to study the high-frequency changes in the ionosphere electron density. Electron precipitation is shown to be a likely cause of the small-scale structures which induce the high-frequency refractive variations in the GPS carrier phase.