Ion composition in the plasma trough and plasma plume derived from a Combined Release and Radiation Effects Satellite magnetoseismic study

摘要

The mass and energy carried in the magnetosphere by heavy ions, O+ in particular, are known to increase as geomagnetic activity increases. However, the ion composition in the magnetosphere has not been fully specified since measurements of the flux of different ion species from the ionospheric thermal energy (below 1 eV) to the ring current energy (above 100 keV) are difficult with single-particle instruments. We used mass density determined by a magnetoseismology technique and the electron density derived from measured plasma wave spectra to investigate the ion composition and total mass density for Combined Release and Radiation Effects Satellite (CRRES) orbit 962 (27?28 August 1991). This orbit occurred during a geomagnetic storm and included afternoon passes through the plasmasphere, the plasma trough, and a plasma plume, where these plasma regions were identified using the electron density n e . In the magnetoseismology analysis, we determined the fundamental frequency of the toroidal standing Alfvén waves f T1 from the electric and magnetic field data and then inferred the corresponding total mass density ρ total at the satellite by solving an MHD wave equation with a realistic magnetic field model and a realistic assumption for the mass distribution along the field line. The value of f T1 changed little when the spacecraft moved between the plasma trough and the plasma plume, implying the dominance of heavy ions in the plasma trough. From the values of n e and ρ total, we derived quantities associated with O+ by assuming that the plasma consisted of three ions, H+, He+, and O+. In the plasma trough, O+ ions are found to carry a number density of ～10 cm?3, ～50% of the number density, and ～90% of the mass density. On the other hand, O+ is found to be much less dominant in the plasma plume. Our results are consistent with DE-1 studies of the formation of an oxygen torus at the outer edge of the H+ plasmapause during geomagnetic active periods and with GEOS-1 and GEOS-2 studies that reported strong dependence of O+ density on geomagnetic activity and on solar extreme ultraviolet flux. In addition, our events indicate that the plasma plume boundary, defined in terms of the number density of electrons or light ions (H+ and He+), may not exhibit similar structure in the total mass density that can be readily detected using magnetoseismology techniques.