Inner magnetospheric plasma pressure distribution and its local time asymmetry

Distribution of plasma pressure in the inner magnetosphere for protons and oxygen ions is constructed using measurements from two particle instruments on AMPTE/CCE covering an energy range of ∼1 keV to 4 MeV. Data within the L shells from 2 to 9 in the equatorial region are sorted into low and high activity levels based on the Kp index. It is found that (1) the pressure peak of protons, the dominant contributor to the total plasma pressure typically, occurs at a lower L shell than that of oxygen ions at all local times, (2) proton pressure is generally higher in the dusk‐midnight sector than in the post‐midnight sector during high Kp , a feature reinforced by the expected local time asymmetry in the magnetic field strength, and (3) a minimum magnetic field region seems to exist at L ∼ 8 in the night sector during high Kp but is apparently absent during low Kp . These results suggest that the plasma pressure peak lies in the dusk‐midnight sector during active periods, in contrast to the plasma pressure distribution inferred from energetic neutral atom observations during active times. Possible resolutions to this apparent discrepancy are suggested. This study also indicates that within L ∼ 9 a minimum magnetic field region does not develop during low Kp but appears at high Kp .