[P-EM13] Inner magnetosphere: Recent understanding and new insights

[PEM13-P23] Meridional distributions of proton plasma and pressure-driven currents in the nightside inner magnetosphere: Arase observations

*Shun Imajo¹, Masahito Nose¹, Satoshi Kasahara², Shoichiro Yokota³, Ayako Matsuoka⁴, Kunihiro Keika², Tomoaki Hori¹, Mariko Teramoto¹, Kazuhiro Yamamoto⁵, Oimatsu Satoshi⁵, Reiko Nomura⁶, Akiko Fujimoto⁷, Iku Shinohara⁴, Yoshizumi Miyoshi¹ (1.Institute for Space-Earth Environmental Research, Nagoya University, 2.Graduate School of Science, The University of Tokyo, 3.Graduate School of Science, Osaka University, 4.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 5.Graduate School of Science, Kyoto University, 6.National Astronomical Observatory of Japan, 7.Kyushu Institute of Technology)

Keywords:meridional distributions, plasma pressure, pressure-driven currents, Arase satellite

We examine the averaged meridional distributions of proton plasma parameters and pressure-driven currents in the nightside (20–04 h magnetic local time) ring current region during disturbed times (-80 < SYM-H^* < -40 nT) using ion measurements covering energy range of 10–180 keV by the Arase satellite. Because the Arase satellite has a large inclination orbit of 31^°, it covers the absolute value of magnetic latitude (|MLAT|) range of 0–40^° and the radial distance of < 6 Re. We find that the plasma pressure decreases significantly with MLAT. The plasma pressure on the same L^* shell at 30^°<|MLAT|<40^° is about 10–60% of that at 0^°<|MLAT|<10^°, and the percentage of decrease is larger on lower L^* shells. The pressure anisotropy which is defined by the perpendicular pressure divided by the parallel pressure decreases with radial distance and shows a weak dependence on |MLAT|. The magnitude of the plasma beta at 30^°<|MLAT|<40^° is one or two order smaller than that at 0^°<|MLAT|<10^°. The relative plasma pressure distribution predicted from the magnetic strength and anisotropy is roughly consistent with the observed plasma pressure for L^*=3.5–5.5. The azimuthal pressure-gradient current derived from the plasma pressure distribution spreads over ~-20^° |MLAT|, while the curvature current is limited in ~0–10^° |MLAT|. We suggest that the latitudinal dependences should be taken into account when considering a temporal evolution of ring current particles from L distributions of successive orbits.