[P-PS05] Recent advances in the science of Venus

[PPS05-08] Life cycle of large cloud discontinuity in Venus' atmosphere as inferred from Akatsuki/IR2 data

*Takehiko Satoh¹, Takao M. Sato², George HASHIMOTO³, Takeshi Imamura⁴, Takeshi Kuroda⁵ (1.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 2.Hokkaido Information University, 3.Okayama University, 4.The University of Tokyo, 5.Tohoku University)

Keywords:Venus clouds, discontinuity, infrared observation, radiative transfer

Observations of Venus' night-side disc in the transparency windows of the CO₂ atmosphere (including 1.74 and 2.3 um wavelengths) allow us to study various atmospheric phenomena in the middle to lower cloud layers of Venus. A sharp and large discontinuity of cloud opacities was observed in the night-side of Venus with the IR2 2-um camera onboard Akatsuki Venus Orbiter (Peralta et al., 2020). We have developed image restoration techniques which successfully recovered the "true" contrast between this enormous cloud cover (ECC) and the adjacent background cloud (BC) regions. By comparing the data in two filters of IR2 (1.735 and 2.26 um) with radiative transfer computations, the differences of cloud structure in the BC and ECC regions were studied. It is found that an increase of Mode 3 particles near the cloud base (~48 km altitude) can reproduce the decreasing radiance from BC to ECC in both filters. Although this is not a unique explanation of how the cloud structure changes from BC to ECC region, this is consistent with an idea that such a discontinuity can be produced by a bore wave near the cloud base. Case studies of different observation dates and implications to the mechanism will be discussed.