[P-EM11] Space Weather and Space Climate

[PEM11-02] Precipitation of auroral electrons accelerated at very high altitudes

★Invited Papers

*今城 峻¹、三好 由純²、風間 洋一³、浅村 和史⁴、篠原 育³、塩川 和夫²、笠原 禎也⁵、笠羽 康正⁸、松岡 彩子¹、Wang Shiang‐Yu³、Tam Sunny⁶、Jun Chae-Woo²、寺本 万里子⁷、栗田 怜¹、土屋 史紀⁸、熊本 篤志⁸、齋藤 幸碩⁸、堀 智昭² (1.京都大学、2.名古屋大学、3.Academia Sinica Institute of Astronomy and Astrophysics、4.宇宙科学研究所、5.金沢大学、6.National Cheng Kung University、7.九州工業大学、8.東北大学)

キーワード：オーロラ加速領域、inverted-V電子、沿磁力線電流、あらせ衛星

The electric potential accelerating auroral electrons has been observed to lie immediately above the ionosphere, at altitudes of a few thousand kilometers. In contrast to these observations, we show that active auroral arcs are powered by electrons accelerated at altitudes reaching greater than 30,000 km, employing high-angular resolution electron observations achieved by the Arase satellite in the magnetosphere and optical observations of aurora by a ground-based all-sky imager. The observed properties and dynamics of electrons at ~30,000 km altitudes and ~30^o dipole magnetic latitudes resemble those of electron potential acceleration reported with low-altitude satellites except that the acceleration region is located at a much higher altitude than previously assumed (Imajo et al., 2021). The accelerated electrons inside the loss cone carried significant net field-aligned current (FAC) density and energy flux into the ionosphere. The height distribution of the acceleration region below the satellite estimated from the frequency of the auroral kilometric radiation was ~3,000–15,000 km altitudes, suggesting that the very-high altitude acceleration region was separated from the lower acceleration region. We also found electrostatic waves of a sub-second time scale with a thin FAC indicated by magnetic deflections. Such electrostatic wave may be generated by the formation of the double layer in the magnetotail above ~30,000 km altitude, and its potential drop contributed significantly (~40–60 %) to the parallel energization of precipitating auroral electrons. Our numerical simulation shows that a potential distribution with a high-altitude double layer at ~42,000 km altitude can be a quasi-neutral solution for that along the magnetic field lines from the ionosphere to the equatorial plane.