Long-term variations of temperature and H2SO4 gas mixing ratio in the Venusian atmosphere studied with Venus Express and Akatsuki radio occultation measurements

It is well known that planetary atmospheres have a wide range of time-scale variations that have a significant impact on the surface environment of the planet. Taking the Earth as an example, long-term scale atmospheric variations such as ENSO and QBO are affecting the Earth's weather and climate. On the other hand, whether such long-term scale atmospheric variations exist in planetary atmospheres other than Earth has not been much studied in the past. A change in this situation has been brought by the recent realization of long-term observations of the Venusian atmosphere by Venus Express and Akatsuki: the presence of significant long-term variations in UV albedo and the zonal mean wind speed at cloud levels have been reported (Peralta et al., 2018; Lee et al., 2019; Khatuntsev et al., 2023; Horinouchi et al., 2024). In addition, the measurements using ground-based telescopes also have indicated the long-term variations in the mixing ratios of H₂O and SO₂ gases at the cloud top level (Encrenaz et al., 2023). In this study, we used the vertical profiles of temperature and H₂SO₄ gas mixing ratio obtained by the Venus Express and Akatsuki radio occultation measurements (e.g., Häusler et al., 2006; Imamura et al., 2017; Ando et al., 2020) and investigated their long-term variations in the low latitude region. As a preliminary result, we found that both temperature and H₂SO₄ gas mixing ratio around the cloud bottom level (46–50 km altitudes) in 2006–2014 obtained from the radio occultation measurements by Venus Express are generally higher than those in 2016–2022 from the Akatsuki data. In addition, the trend of the annual variation of H₂SO₄ gas mixing ratio almost follows that of its saturated mixing ratio which is mainly determined by the temperature. In the presentation, we will also discuss the long-term variations of temperature in a wider vertical range from the mesosphere down to the sub-cloud region (~40–85 km altitudes).