[P-PS04] Advancing the science of Venus in the golden age of exploration

[PPS04-P10] Long-term variation of the Venus atmosphere using infrared and ultraviolet images

*Mizuho WATANABE¹, Takeshi Imamura¹, Makoto Taguchi², Toru Kouyama³, Atsushi Yamazaki⁴ (1.The University of Tokyo , 2.Rikkyo University, 3.National Institute of Advanced Industrial Science and Technology, 4.Institute of Space and Astronautical Science / Japan Aerospace Exploration Agency)

The climate of Venus is thought to be maintained by the interaction among dynamics, chemistry and radiative transfer. Sulfuric acid clouds have a large albedo and a large cooling effect on the atmosphere. Unknown absorbers near the cloud tops of Venus have a broad absorption band from UV to visible wavelengths with a peak around 360 nm, and are thought to greatly contribute to the absorption of solar energy(Lee et al.,2019). Thermal tides excited by the periodic heating by sunlight near the cloud tops might drive the meridional circulation as well as the superrotation of the atmosphere(Horinouchi et al.,2020). Therefore, the sulfuric acid clouds should play major roles in climate control. Although the feedbacks that maintain the climate are still unclear, long-term atmospheric variations that have been observed should provide clues; it will be possible to estimate the feedbacks among the physical quantities by studying the relationships among them. However, there have been no long-term, comprehensive observations of the entire Venusian atmosphere including the equatorial region where the majority of the solar energy is provided. Therefore, we do not know what kind of time-scale variations exist in the Venusian atmosphere and what kind of physical processes contribute to these variations.
In this study, we explore the relationship between the multi-year scale variations of various physical quantities at the cloud-top altitude of Venus by analyzing infrared and ultraviolet images obtained by LIR and UVI, respectively, onboard AKATSUKI over a long period of time. In particular, we obtained long-term time series of the cloud-top temperature, the UV albedo, the meridional velocity, and the amplitudes of thermal tides, and calculated correlation coefficients among the observables. The results show that the observed quantities show quasi-periodic variations of about 4 to 6 Venus years, which is shorter than the 10 Venus-year scale variations reported in the previous studies. Based on the calculated correlation coefficients, we could discuss, for example, the effect of albedo change on the temperature, the cloud particle transport by the Hadley circulation, and the variation in the meridional circulation due to the change in the amplitudes of thermal tides. To further confirm the processes, longer-term observations and information on lower altitudes are needed.