Publication Date:
2014-08-08
Description:
We investigated mass density ρ m and O + concentration η O + ≡ n O + / n e (where n O + and n e are the O + and electron density, respectively)during two events, one active and one more quiet. We found ρ m from observations of Alfvén wave frequencies measured by the Geostationary Operational Environmental Satellites (GOES), and we investigated composition by combining measurements of ρ m with measurements of ion density n MPA,i from the Magnetospheric Plasma Analyzer (MPA) instrument on Los Alamos National Laboratory (LANL) spacecraft or n e from the Radio Plasma Imager (RPI) instrument on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft. Using a simple assumption for the He + density at solar maximum based on a statistical study, we found η O + values ranging from near zero to close to unity. For geostationary spacecraft that co-rotate with the earth, sudden changes in density for both ρ m and n e often appear between dusk and midnight MLT, especially when Kp is significantly above zero. This probably indicates that the bulk (total) ions have energy below a few keV and that the satellites are crossing from closed or previously closed to open drift paths. During long periods that are geomagnetically quiet, the mass density varies little, but n e gradually refills leading to a gradual change in composition from low density plasma that is relatively cold and heavy (high average ion mass M ≡ ρ m / n e ) to high density plasma that is relatively cold and light (low M ) plasmasphere-like plasma. During active periods we observe a similar daily oscillation in plasma properties from the dayside to the nightside, with cold and light high density plasma (more plasmasphere-like) on the dayside, and hotter and more heavy low density plasma (more plasmasheet-like) on the nightside. The value of n e is very dependent on whether it is measured inside or outside a plasmaspheric plume, while ρ m is not. All of our results were found at solar maximum; previous results suggest that there will be much less O + at solar minimum under all conditions.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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