Radial profiles of quiet time magnetospheric parameters

Abstract

Magnetospheric plasma parameters during geomagnetically quiet conditions are studied with ion measurements covering energies from ∼1 keV to ∼4 MeV from the Charge Composition Explorer. It is found that the observed quiet time plasma pressure in the midnight sector is comparable (within a factor of 2) to that deduced by Spence et al. (1989) and Kan et al. (1992) from inverting the magnetic field models of Tsyganenko and Usmanov (1982) and Tsyganenko (1987). The radial profile of the total plasma pressure shows a peak generally at L ≈ 3 to 4 and decreases from L ≈ 4 to L ≈ 9 rather monotonically. No large‐scale earthward decrease in plasma pressure occurs in these outer L shells, contrary to some theoretical expectation for the magnetospheric closure of the region 2 field‐aligned current system. The plasma pressure within L ∼ 9 is generally anisotropic with a larger pressure component perpendicular to the magnetic field than parallel to the field. This anisotropy tends to increase with smaller L. While the anisotropy is always below the fire hose instability threshold during all quiet time passes studied, it is near the mirror instability threshold in the midnight sector at large L shells due mostly to the large value of plasma beta (>1) there. The radial profile of the azimuthal volume current density is also computed, with the result suggesting that the current density associated with the quiet time ring current population is distributed broadly over the L shells between L ≈ 3.5 and L ≈ 7 with average values in the range of ∼1 to 4 nA/m² and peak values in the range of ∼4 to 8 nA/m². Another current region situated beyond L ≈ 7, which may be related to the inner portion of the quiet time cross‐tail current, is also apparent and has average and peak current density values similar to those of the ring current.