Multi-spacecraft observations of oxygen ion flows in the polar topside ionosphere and lower magnetosphere.

摘要

This dissertation is comprised of analyses of the combined measurements from the Thermal Ion Dynamics Experiment (TIDE) onboard the Polar spacecraft at perigee altitudes near 5000 km, and thermal plasma detector arrays on multiple DMSP spacecraft near 840 km altitude, and numerical simulations of ion transport from the ionospheric F-region to the lower magnetosphere at high latitudes. We first describe the data analysis techniques used in the dissertation. We display near-simultaneous observations of topside O+ parallel flows for four periods of measurements by the Polar and DMSP satellites during April 1996. The O+ densities and flux were characteristically 1--10 cm-3 and 105---10 7 cm-2s-1 at 5000 km altitude, and 103--104 cm-3 and 107--109 cm-2s -1 at 840 km altitude, respectively. The extent of downward flows at 840 km altitude is wider than that at 5000 km. For some instances, downward flows are measured at low altitudes while upward flows are observed at high altitudes on possibly the same magnetic flux tube. We also report observations of O+ density troughs at high latitudes near 5000 km altitude. Our statistical results show that the O+ density trough was always located on the nightside portion of the polar cap magnetosphere/ionosphere. The trough occurrence was strongly dependent on season and solar zenith angle, generally anti-correlated with solar wind dynamic pressure, and relatively independent of geomagnetic Kp, IMF Bz, and By conditions.; In the simulation portion of this dissertation, we use the Dynamic Fluid-Kinetic (DyFK) model to simulate ion transport along a flux tube. We considered one case in which the simulated density and velocity altitude profiles generally bracketed the near-simultaneous observations by Polar and DMSP along the same field line, except that the observed downward velocities at 840 km altitude were larger in magnitude than those in the transport simulation, and the simulated densities were several times higher than those observed at both altitudes. We also found that by allowing a flux tube to drift along a realistic convection trajectory for 6 hours in which the F-region portion resided within darkness, an O+ density trough at 5000 km altitude developed in our simulation.