[P-EM15] Dynamics of Magnetosphere and Ionosphere

[PEM15-01] Effect of field line curvature scattering on precipitation and isotropy of energetic protons in the magnetosphere

*今城 峻¹、三好 由純²、笠原 慧³、横田 勝一郎⁴、松岡 彩子¹、桂華 邦裕³、堀 智昭²、篠原 育⁵、塩川 和夫² (1.京都大学、2.名古屋大学、3.東京大学、4.大阪大学、5.宇宙科学研究所)

キーワード：等方境界、高エネルギープロトン、磁力線曲率散乱、あらせ衛星

Although low-altitude observations have shown that isotropic precipitation of energetic protons is caused by the field-line curvature scattering, it is not clear whether this process causes isotropy of the protons observed in the magnetosphere at L > ~9. In this study, we have distinguished the low-latitude boundaries of the loss cone filling and the isotropic distribution of energetic protons in an energy range of 10-180 keV/q using middle-altitude off-equatorial observations (3-5 Re geocentric distances) made by the Arase satellite. The isotropic distribution boundary is defined by the ratio of proton fluxes at pitch angles of 0-45 degrees and 45-90 degrees for the northern hemisphere. The latitude of the isotropic distribution boundary had an energy dependence such that higher energy protons stay isotropic up to lower latitudes, implying that protons were isotropized by field line curvature scattering. Around the isotropic distribution boundary, the downward loss cone (within 5 degrees from the ambient magnetic field) was filled, while the corresponding upward loss cone was empty due to atmospheric loss. The low-latitude boundary of the loss cone filling tended to be located at 0.1-0.5 degrees lower latitude from the isotropic distribution boundary. A numerical calculation with the centrifugal impulse model shows the flux distribution modified by field-line curvature scattering can explain such a latitude difference in the low-latitude boundaries between the loss cone filling and the isotropic distribution. These results suggest that the effective pitch angle scattering occurred only near the loss cone on the field line with a larger equatorial curvature radius (lower latitude), resulting in loss cone filling, whereas it occurred in the entire pitch angle on the field line with a smaller equatorial curvature radius (higher latitude), resulting in the isotropic distribution.