[P-EM10] Dynamics of Magnetosphere and Ionosphere

[PEM10-13] On the relationship between energy input to the ionosphere and the ion outflow flux under different solar zenith angles

*北村 成寿¹、関 華奈子¹、桂華 邦裕¹、西村 幸敏²、堀 智昭³、平原 聖文³、Lund Eric^4,5、Kistler Lynn⁴、Strangeway Robert⁶ (1.東京大学大学院 理学系研究科 地球惑星科学専攻、2.ボストン大学 電気情報工学専攻 宇宙物理学研究センター、3.名古屋大学 宇宙地球環境研究所、4.ニューハンプシャー大学 地球海洋宇宙研究所、5.カリッジブルックサイエンティフィック、6.カリフォルニア大学ロサンゼルス校 地球惑星宇宙科学専攻)

キーワード：イオン流出、FAST衛星、オーロラ帯、カスプ

The ionosphere is an important source for magnetospheric plasma, particularly for heavy ions with low charge states. We investigate the effect of solar illumination on the number flux of ion outflow using data obtained by the Fast Auroral SnapshoT (FAST) satellite at 3000–4150 km altitude from 7 January 1998 to 5 February 1999. We derive empirical formulas between energy inputs and outflowing ion number fluxes for various solar zenith angle ranges. It is found that the outflowing ion number flux under sunlit conditions increases more steeply with increasing electron density in the loss cone or with increasing precipitating electron density (above 50 eV), compared to the ion flux under dark conditions. Under ionospheric dark conditions, weak electron precipitation can drive ion outflow with small averaged fluxes (about 10⁷ cm²/s). The slopes of relations between the Poynting fluxes and outflowing ion number fluxes show no clear dependence on the solar zenith angle. Intense ion outflow events (larger than 10⁸ cm²/s) occur mostly under sunlit conditions (solar zenith angle < 90°). Thus, it is presumably difficult to drive intense ion outflows under dark conditions, because of a lack of the solar illumination (low ionospheric density and/or small scale height owing to low plasma temperature).