[P-EM16] 大気圏・電離圏

[PEM16-08] 高分解能Ca⁺ライダーによって観測されたスポラディックE層の微細構造

*江尻 省^1,2、中村 卓司^1,2、津田 卓雄³、西山 尚典^1,2、阿保 真⁴、高橋 透¹、津野 克彦⁵、川原 琢也⁶、小川 貴代⁵、和田 智之⁵ (1.国立極地研究所、2.総合研究大学院大学、3.電気通信大学、4.首都大学東京、5.理化学研究所光量子工学研究センター、6.信州大学工学部)

キーワード：共鳴散乱ライダー、カルシウムイオン密度、スポラディックE層、微細構造、中間圏・下部熱圏領域

The vertical fine structures and the time evolution of plasma irregularities in the sporadic E (E_s) layer were observed via calcium ion (Ca⁺) density measurements using a resonance scattering lidar with a high time-height resolution (5 s and 15 m) at Tachikawa (35.7°N, 139.4°E) on December 24, 2014. The observation successfully provided clearer fine structures of plasma irregularities, such as quasi-sinusoidal variation, localized clumps, “cats-eye” structures, and twist structures, in the sporadic Ca⁺ (Ca⁺_s) layers at around 100 km altitude. These fine structures suggested that the Kelvin-Helmholtz (K-H) instabilities occurred in the neutral atmosphere whose density changed temporarily or spatially. The maximum Ca⁺ density in the Ca⁺_s layer was two orders of magnitude smaller than the maximum electron density estimated from the critical frequency (f_oE_s) observed by the ionosonde at Kokubunji (35.7°N, 139.5°E) simultaneously. The correlation showed a strong positive correlation with a coefficient of 0.91. These results suggest that Ca⁺ contributes forming the E_s layer as well as major metallic ions Fe⁺ and Mg⁺ in the lower thermosphere. Moreover, the formation of a new Ca⁺_s layer at 110 km and the upward motions of the Ca⁺_s layers at 100 km and 110 km were observed just after the sunrise time at the conjugation point and before the local sunrise. Although the presence or absence of a causal relationship with the sunrise time was not clear, a possible explanation for the formation and the upward motions of the Ca⁺_s layers was the occurrence of strong eastward winds at around 100 km, rather than the enhancement of the eastward electric field.