[P-PS06] Science of Venus: Venus Express, Akatsuki, and beyond

[PPS06-11] Dayside cloud top structure of Venus retrieved from Akatsuki IR2 observations

*佐藤 隆雄¹、佐藤 毅彦²、佐川 英夫³、眞子 直弘⁴、リー ヨンジュ⁵、村上 真也²、小郷原 一智⁶、はしもと じょーじ⁷、笠羽 康正⁸、山崎 敦²、山田 学⁹、渡部 重十¹、今村 剛¹⁰、中村 正人² (1.北海道情報大学、2.宇宙航空研究開発機構宇宙科学研究所、3.京都産業大学、4.千葉大学、5.ベルリン工科大学、6.滋賀県立大学、7.岡山大学、8.東北大学、9.千葉工業大学、10.東京大学)

キーワード：金星、大気、雲構造、赤外観測、放射伝達、あかつき

We describe the dayside cloud top structure of Venus as retrieved from 93 images acquired at a wide variety of solar phase angles (0–120 deg) using the 2.02-μm channel of the 2-μm camera (IR2) onboard the Venus orbiter, Akatsuki, from April 4 to May 25, 2016. Since the 2.02-μm channel is located in a CO₂ absorption band, the sunlight reflected from Venus allowed us to determine the cloud top altitude corresponding to a unit aerosol optical depth at 2.02 μm. First, the observed solar phase angle dependence and the center-to-limb variation of the reflected sunlight in the region equatorward of 30 deg were used to construct a spatially averaged cloud top structure characterized by cloud top altitude , Mode 2 modal radius , and cloud scale height , which were 70.4 km, 1.06 μm, and 5.3 km, respectively. Second, cloud top altitudes at individual locations were retrieved on a pixel-by-pixel basis with an assumption that and were uniform for the entire planet. The latitudinal structure of the cloud top altitude was symmetric with respect to the equator. The average cloud top altitude was 70.5 km in the equatorial region and showed a gradual decrease of ~2 km by the 45 deg latitude. It rapidly dropped at latitudes of 50–60 deg and reached 61 km in latitudes of 70–75 deg. The average cloud top altitude in the region equatorward of 30 deg showed negligible local time dependence, with changes up to 1 km at most. Local variations in cloud top altitude, including stationary gravity wave features, occurred within several hundreds of meters. Although long zonal or tilted streaky features poleward of ~45 deg were clearly identifiable, features in the low and middle latitudes were usually subtle. These did not necessarily appear as local variations at the cloud top level, where mottled and patchy UV patterns were observed, suggestive of convection and turbulence at the cloud top level.