| The physical structure of the different regions in the magnetosphere is determined by the interaction of electric fields, magnetic fields, and charged plasma particles. In particular, the density of particles is important because it influences the propagation of plasma waves, affects the ability of the plasma to carry currents, and provides a mechanism for storing kinetic energy. The Polar spacecraft, owing to the large spatial and temporal coverage of its orbit, is ideal for studying the seasonal and solar cycle variations of the plasma density at geocentric distances less than 9 RE . In the analysis presented here, the spacecraft floating potential is measured using the Polar electric field instrument (EFI) and employed to infer the electron plasma density. These density measurements allow the examination of the characteristics of plasma density in different regions of the magnetosphere, the correlation of the plasma density with processes associated with the visible aurora, and the variation of the ionosphere as a source of magnetospheric plasma.; The results presented here show strong seasonal plasma density variations at altitudes far above the ionosphere. These variations are directly correlated to changes in the illumination of the ionosphere below and are seen in both the polar cap and auroral oval. Altitude distributions of plasma in the auroral acceleration region show these density changes are correlated with the occurrence of large electric fields, and thus, with the generation of auroral displays. In the polar cap, these measurements show a dependence of the plasma density at 2 RE geocentric on both solar zenith angle and the solar cycle. These measurements provide useful boundary conditions for modeling different acceleration mechanisms responsible for the polar wind and can be compared to simulation results to determine the relative importance of physical processes on polar wind outflow. |
