[P-EM13] 内部磁気圏

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

*今城 峻¹、能勢 正仁¹、笠原 慧²、横田 勝一郎³、松岡 彩子⁴、桂華 邦裕²、堀 智昭¹、寺本 万里子¹、山本 和弘⁵、生松 聡⁵、野村 麗子⁶、藤本 晶子⁷、篠原 育⁴、三好 由純¹ (1.名古屋大学宇宙地球環境研究所、2.東京大学理学系研究科、3.大阪大学理学研究科、4.宇宙科学研究所、5.京都大学理学研究科、6.国立天文台、7.九州工業大学)

キーワード：子午面分布、プラズマ圧、圧力励起電流、あらせ衛星

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.