[P-PS05] Recent advances of Venus science

[PPS05-P06] Enormous cloud cover as seen by Akatsuki/IR2 on the night-side disk of Venus

*佐藤 毅彦^1,2、Vun Choon Wei²、佐藤 隆雄³、堀之内 武⁴、はしもと じょーじ⁵、McGouldrick Kevin⁶ (1.宇宙航空研究開発機構・宇宙科学研究所、2.総合研究大学院大学・宇宙科学専攻、3.北海道情報大学・経営情報学部、4.北海道大学・地球環境科学研究院 、5.岡山大学・理学部地球科学科、6.LASP, University of Colorado Boulder)

キーワード：あかつきIR2、金星夜面、雲濃度

Night-side observations in transparency windows of CO₂ atmosphere of Venus allow visualizing inhomogeneous clouds in ~50 to ~60 km altitudes. The IR2 camera on board Akatsuki captured such images at 1.735, 2.26, and 2.32 um wavelengths. In the IR2 night-side data, an enormous cloud of greater opacity and very sharp edge in the front (western end) is seen repeatedly. This feature seems to encircle the planet with a period of ~4.5 days (Peralta).
Although interesting, property of particles in this feature was not studied before due to difficulty of photometric measurements in IR2 night-side data. This difficulty comes from a combination of the intense day crescent and extended tail of IR2 point-spread function (multiple reflection in the detector is the cause). We have developed a technique to restore the contrast of the night-side data by deconvolution so that the photometric studies can be done with IR2 night-side data.
The data acquired on 18th and 27th August 2016 are analyzed. We have performed a series of radiative transfer computations to reproduce both 1.735- and 2.26-um opacities in this enormous cloud. Properties of the cloud partices will be presented and possible mechanism of this remarkable phenomenon will be discussed.