[P-EM12] Dynamics of the Inner Magnetospheric System

[PEM12-05] Deflection of upflowing ion beams by a converging electric field in the auroral flux tube: Alternative explanation for mass-dependent beam width

★Invited Papers

*今城 峻¹、三好 由純¹、浅村 和史²、篠原 育²、能勢 正仁¹、笠原 禎也³、笠羽 康正⁴、松岡 彩子⁵、堀 智昭¹、小路 真史¹、中村 紗都子¹、寺本 万里子⁶ (1.名古屋大学、2.JAXA 宇宙科学研究所、3.金沢大学、4.東北大学、5.京都大学、6.九州工業大学)

キーワード：上昇イオンビーム、オーロラ加速域、電場ドリフト、質量依存性

The pitch angle distribution of O⁺ upflowing ion beams wider than that of H⁺ beams has often been explained by mass-dependent perpendicular heating. Contrary to this conventional explanation, here we show that the mass-dependent pitch angle distribution of the ion beams can arise from a deflection of ion beams by a converging electric field in the auroral flux tube. On May 20, 2017, the Arase (ERG) satellite at ~32,000 km altitude and 1.1 h magnetic local time observed two consecutive ion beam events associated with an inverted-V shape spectrum in an energy range of ~1–8 keV. Magnetic and electric field variations during the ion beams corresponded to a relative equatorward motion of upward-field aligned current and converging perpendicular electric field, respectively, in the flux tube above the auroral acceleration region. Examining the gyro-phase distribution of the beam component, we found that the ion beams were significantly deflected in an east-west direction. The direction of ion flux was reversed from westward to eastward at the same time concurrently with the electric field reversal from outward to inward, indicating the ExB drift by the converging electric field causes this beam deflection. The difference of beam width between H⁺ and O⁺ tended to be large when the magnitude of the electric field was large. The field-aligned velocity of the H⁺ beams is ~4 times higher than that of the O⁺ beams, while the perpendicular beam velocity is consistent with the ExB drift. Because of the slower field-aligned velocity of O⁺ beams, they are more easily deflected by the ExB drift compared with the light ions like H⁺. This non-negligible perpendicular drift can result in an overestimation of beam width when it is estimated from a pitch angle distribution averaged over the entire gyro phase in a satellite-centered coordinate system.