Evolution of mass density and O plus concentration at geostationary orbit during storm and quiet events

摘要

abstract_textpWe investigated mass density rho(m) and O+ concentration eta(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 rho(m) from observations of Alfven wave frequencies measured by the GOES, and we investigated composition by combining measurements of rho(m) with measurements of ion density n(MPA, i) from the Magnetospheric Plasma Analyzer (MPA) instrument on Los Alamos National Laboratory spacecraft or n(e) from the Radio Plasma Imager instrument on the Imager forMagnetopause-to-Aurora Global Exploration spacecraft. Using a simple assumption for the He+ density at solar maximum based on a statistical study, we found eta(O+) values ranging from near zero to close to unity. For geostationary spacecraft that corotate with the Earth, sudden changes in density for both rho(m) and n(e) often appear between dusk and midnight magnetic local time, 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 ne gradually refills leading to a gradual change in composition from low-density plasma that is relatively cold and heavy (high-average ion mass M = rho(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 plasma sheet-like) on the nightside. The value of ne is very dependent on whether it is measured inside or outside a plasmaspheric plume, while rho(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./p/abstract_text