Evolution of mass density and O+ concentration at geostationary orbit during storm and quiet events. Issue 8 (22nd August 2014)

• Record Type: Journal Article
• Authors: Denton, R. E.; Takahashi, K.; Thomsen, M. F.; Borovsky, J. E.; Singer, H. J.; Wang, Y.; Goldstein, J.; Brandt, P. C.; Reinisch, B. W.
• Is Part Of: Journal of geophysical research. Volume 119:Issue 8(2014:Aug.)
• Page Start: 6417
• Page End: 6431
• Publication Date: 2014-08-22
• Subjects: Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1002/2014JA019888
• Languages: English
• ISSNs: 2169-9380

Abstract

<abstract abstract-type="main" id="jgra51229-abs-0001"> <title>Abstract</title> <p id="jgra51229-para-0001">We investigated mass density <italic>ρ</italic><sub>m</sub> and O+ concentration <italic>η</italic><sub>O+</sub>≡<italic>n</italic><sub>O+</sub>/<italic>n</italic><sub>e</sub> (where <italic>n</italic><sub>O+</sub> and <italic>n</italic><sub>e</sub> are the O+ and electron density, respectively) during two events, one active and one more quiet. We found <italic>ρ</italic><sub>m</sub> from observations of Alfvén wave frequencies measured by the GOES, and we investigated composition by combining measurements of <italic>ρ</italic><sub>m</sub> with measurements of ion density <italic>n</italic><sub>MPA, i</sub> from the Magnetospheric Plasma Analyzer (MPA) instrument on Los Alamos National Laboratory spacecraft or <italic>n</italic><sub>e</sub> from the Radio Plasma Imager instrument on the Imager for Magnetopause‐to‐Aurora Global Exploration spacecraft. Using a simple assumption for the He+ density at solar maximum based on a statistical study, we found <italic>η</italic><sub>O+</sub> values ranging from near zero to close to unity. For geostationary spacecraft that corotate with the Earth, sudden changes in density for both <italic>ρ</italic><sub>m</sub> and <italic>n</italic><sub>e</sub> 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 <italic>n</italic><sub>e</sub> gradually refills leading to a gradual change in composition from low‐density plasma that is relatively cold and heavy (high‐average ion mass <italic>M</italic> ≡ <italic>ρ</italic><sub>m</sub>/<italic>n</italic><sub>e</sub>) to high‐density plasma that is relatively cold and light (low <italic>M</italic>) 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 <italic>n</italic><sub>e</sub> is very dependent on whether it is measured inside or outside a plasmaspheric plume, while <italic>ρ</italic><sub>m</sub> 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>