Tatsuro Iwanaka

IntroductionThe sulfuric acid clouds of Venus located at altitudes of 50–70 km play an important role in the energy budget of Venus. While this cloud layer reflects approximately 80% of the incident solar radiation, it absorbs most of the remaining energy, thereby determining the thermal structure of the whole atmosphere [1]. Sulfur dioxide, the chemical precursor of these clouds, is transported from the lower cloud layer and converted into sulfuric acid through photochemical reactions at the cloud top [2]. Therefore, clarifying the spatial distribution and transport processes of sulfur dioxide is essential for understanding the mechanisms that form and maintain the Venusian clouds. Furthermore, an unidentified UV absorber dominates solar energy absorption in the ultraviolet range and significantly influences dynamical structures, such as superrotation and thermal tides, by varying solar heating rates. There are likely complex feedbacks where atmospheric motions vary the distribution of these materials, which in turn induces changes in absorption and reflectance, modulating the solar energy supply.These atmospheric motions range hierarchically from planetary-scale waves to local turbulence across various spatio-temporal scales. However, polar-orbiting observations so far have been limited by spatio-temporal continuity, making it difficult to continuously capture short-term fluctuations and global-scale dynamics. In contrast, the Japanese Venus orbiter, Akatsuki, has conducted high-frequency observations of the entire Venusian disk at 2-hour intervals using the Ultraviolet Imager (UVI) from a low-inclination orbit. Our previous research using approximately 15,000 dual-channel image pairs (283 nm and 365 nm) revealed that planetary-scale waves, such as thermal tides and Kelvin waves, excite fluctuations in the distribution of these materials [3]. However, these waves alone primarily induce periodic oscillations. The net transport that effectively carries materials to the cloud top is thought to be driven by the complex combinations of waves, chemical reactions of materials lifted upward, and dynamical processes such as small-scale turbulence and diffusion. In this study, we performed a detailed periodicity analysis and morphological discussions using snapshots to elucidate this material supply mechanism.MethodsWe analyzed over 15,000 pairs of 283- and 365-nm UV images captured by Akatsuki/UVI. These images are Level 3b data products, consisting of radiance maps calibrated on a longitude-latitude grid (0.125-degree resolution). To reduce computational costs and improve the signal-to-noise ratio, the data were spatially binned to a 1-degree resolution. By comparing the observed reflectance distributions at 283 nm and 365 nm with modeled reflectances generated by radiative transfer calculations, we utilized an iterative algorithm to determine the optimal pairs of the sulfur dioxide mixing ratio and the imaginary part of the refractive index of the clouds, a proxy for the unidentified absorber. These retrievals were performed for the entire dataset of 15,000 image pairs.ResultsA longitude-time two-dimensional Lomb-Scargle analysis was conducted on the equatorial distributions of sulfur dioxide and the unidentified absorber (Fig. 1). The results showed that for short-term fluctuations of less than 50 days, a westward-propagating wavenumber-1 structure with a 4-day period was most dominant, likely reflecting vertical atmospheric oscillations associated with equatorial Kelvin waves. Wavenumber-2 components were also identified. On longer timescales of approximately 200 days, signals corresponding to the semi-diurnal thermal tide (58-day period) and eastward-propagating wavenumber-2 components appeared strongly. The signals in the periodogram were not confined to specific peaks but were distributed across a broad frequency band, suggesting that the superposition of various wave components drives effective material transport.In snapshots taken every two hours, the sulfur dioxide distribution, unlike that of the unidentified absorber, showed dark Y-shape structures [4]. This captures the dynamics where sulfur dioxide upwelling in the equatorial region is photochemically depleted while being transported to mid-to-high latitudes by meridional circulation and latitudinal shear of superrotation, providing evidence for the irreversible supply of sulfur dioxide to the cloud top. Furthermore, we observed the splitting of single concentration regions of sulfur dioxide as they moved with the superrotation (Fig. 2). This phenomenon is difficult to explain by the linear superposition of waves alone, suggesting that such non-linear processes contribute to the net transport of materials.Finally, we investigated the relationship between the long-term temporal variations of the cloud-tracked zonal wind speed [5] and the abundance of the unidentified absorber, which can influence solar heating rates. The results showed a negative correlation between these two parameters on a decadal scale, while a strong positive correlation was observed on a scale of 2–3 Venusian years. This suggests the existence of a feedback between material distribution and dynamics through radiative heating.Fig 1. 2D Lomb-Scargle periodograms of equatorial sulfur dioxide and unidentified absorber. The dashed lines indicate the phase velocity of the Kelvin wave. Fig 2. A sequence of the maps cloud-top sulfur dioxide with 2-hour intervals from 23:01 on April 8, 2018 to 21:01 on April 9. The panel numbers indicate the temporal order.References[1] Titov et al., 2007, AGU Geophys. Monogr. Ser., 176, 121-138.[2] Mills and Allen, 2007, Planet. Space Sci., 55, 1729-1740.[3] Iwanaka et al., 2025, J. Geophys. Res. Planets, 130, e2024JE008775.[4] Rossow et al., 1980, J. Geophys. Res., 85, 8107-8128.[5] Horinouchi et al., 2024, J. Geophys. Res. Planets, 129, e2023JE008221.

Venus is often regarded as Earth’s “sister planet”, yet its atmosphere is fundamentally different, being dominated by CO2 (~96%). How such a CO2-rich atmosphere is chemically maintained remains a central unresolved problem in Venus science [1]. In the upper atmosphere, CO2 is dissociated by ultraviolet radiation into CO and O, while the direct recombination reaction, CO + O → CO2, is inefficient because it is spin-forbidden. Therefore, additional chemical pathways are required to explain the observed scarcity of CO and O2 near the cloud-top region. Proposed mechanisms include ClOx catalytic cycles and heterogeneous chemistry on aerosol particles, but these processes remain poorly constrained by observations. In this context, measuring or constraining O2 at the cloud top provides an important test of CO2 recycling chemistry. Previous searches for O2 near the cloud-top region of Venus have been limited to a small number of high-resolution visible spectroscopic observations conducted in 1974, 1982, and 1995 [2-4]. These studies reported upper limits of 3 ppm for the 1974 and 1995 observations and 0.3 ppm for the 1982 observation. Since then, no dedicated observational update has been reported, despite major improvements in telescope aperture, spectrograph performance, and data analysis techniques. Here we present a new search for molecular oxygen at the cloud top of Venus using the High Dispersion Spectrograph (HDS) on the Subaru Telescope. Subaru/HDS provides very high spectral resolution, R ~ 160,000, together with broad spectral coverage, allowing us to combine multiple O2 absorption lines and improve sensitivity to the extremely weak Venusian signal. The observation was performed on 17 June 2025 under a favorable Earth-Venus Doppler-shift geometry, which helps separate Venusian O2 features from telluric absorption. Although the total integration time was only a few minutes, the large aperture of Subaru enabled spectra with sufficiently high signal-to-noise ratio for this search. In the presentation, we compare the observed spectra with radiative transfer calculations including both terrestrial and Venusian absorption components, and report the resulting constraint on the cloud-top O2 abundance.References: [1] Yung & Demore, 1982, Icarus, 51, 199-247. [2] Traub and Carleton, 1974, in Exploration of the Planetary System, 223-228. [3] Trauger and Lunine, 1983, Icarus, 55, 272-281. [4] Mills, 1999, J. Geophys. Res. 104, 30757–30763.

BackgroundThe cloud layer of Venus, composed of sulfuric‑acid aerosols, plays a key role in controlling the solar energy input of the atmosphere through its effect on planetary albedo. To clarify the sequence of processes whereby sulfur dioxide (SO2), the chemical precursor of the clouds, is transported to the cloud‑top region and photochemically converted to sulfuric acid, it is essential to understand the detailed temporal and spatial distribution of SO2 in detail. Observations have been conducted with both ground‑based telescopes [1] and spacecraft [2, 3]; however, these measurements are spatially sparse and temporally sporadic. As a result, the relationship between SO2 variability and atmospheric activities, as well as the evaluation of sulfur cycle and cloud formation scenarios, remains unresolved. The ultraviolet imager (UVI) onboard the Japanese Venus orbiter Akatsuki [4, 5] captures full‑disk images of Venus in two narrowband filters: 283 nm, centered on a SO2 absorption band, and 365 nm, where absorption by an unidentified UV absorber dominates. UVI acquires nearly simultaneous two‑wavelength disk-resolved images every two hours, and its temporal and spatial resolution offers the potential to relate the variation of SO2 and atmospheric dynamics from planetary‑scale waves down to mesoscale phenomena. However, the absorptions of SO2 and the unidentified absorber are mixed in these two‑wavelength images, making it difficult to analyze the detailed variation of SO2 so far. In this study, we separated the absorption contributions of SO2 and the unidentified absorber. Dataset and Retrieval methodsIn this study, we analyzed over 15,000 pairs of 283 and 365 nm UV images taken by Akatsuki/UVI. These UV images are Level 3b data products, which are calibrated radiance maps in the longitude-latitude coordinate (0.125 degree grid). To reduce computational cost, the data were spatially binned to a resolution of 1 degree. We compared the observed reflectance distributions at 283 and 365 nm with simulated reflectance generated by the radiative transfer calculations, and drive the best-fitted pair of the mixing ratio of SO2 and the imaginary part of the cloud refractive index (ni) as a proxy of the unidentified absorber by an iterative algorithm. Additionally, we performed sensitivity tests to examine the effects of cloud altitude and wavelength dependence of ni on the retrieved results. ResultsWe first composited all the data from 2015 to 2022 and found that the SO2 mixing ratio peaks at the equator, decreases toward mid-latitudes, and exhibits a local maximum at 14–15 h local time. We interpret this distribution as a result of the equatorial upwelling in the Hadley-type meridional circulation and vertical movement caused by thermal tides with wavenumber 2, which lifts SO2-rich air from the lower altitudes to the cloud top. This pattern is consistent with the results of a general circulation model coupled with photochemistry [6] and previous Venus Express/SPICAV-UV observations [3], at least with respect to latitude.The longitude-time Hovmöller diagram (Figure 1) shows a consistent feature that moves with a period of 4 days, suggesting the dynamical influence of the Kelvin wave. On a multi-year time scale, the mean SO2 mixing ratio increased from 2016 to 2019 and decreased by 2022 (Figure 2), in agreement with ground-based infrared observations [1]. The long-term variations of the unidentified absorber show a similar trend to those of SO2. These variations are also correlated with changes in the mean zonal wind derived from Akatsuki cloud tracking data [7]. Because the effect of absorption by the unidentified absorber extends into the visible wavelengths where solar irradiance is strong [8], these variations suggest that enhanced solar heating may intensify thermal tides, leading to dynamical feedback that further amplifies wind speeds and further enhances SO2 transport. In this presentation, we highlight these newly analyzed variations across multiple temporal and spatial scales. Figure 1: Hovmöller diagram of SO2 mixing ratio from Feburuary 22 to Jun 2, 2018. Figure 2: Long-term variation of the mean SO2 mixing ratio (orange) and ni (blue) with 1-σ standard deviations (shaded). VY stands for the Venusian year from December 15, 2015 References[1] Encrenaz T. et al., A&A, 674, A199 (2023) [2] Esposito LW et al., JGR-Atmospheres, 93, 5267 (1988) [3] Marcq E. et al., Icarus, 335, 113368 (2020) [4] Nakamura M. et al., Earth Planet. Space, 68, 75 (2016) [5] Yamazaki A. et al., Earth Planet. Space, 70, 23 (2018) [6] Stolzenbach A. et al., Icarus, 395, 115447 (2023) [7] Horinouchi T. et al., JGR–Planets, 129, e2023JE008221 (2024) [8] Moroz VI et al., Sov. Phys. Usp., 28, 524 (1985)

And the unidentified UV absorber are major absorbers in the near‐ultraviolet region in the Venusian atmosphere and influence the climate system. We developed a method to retrieve and the imaginary part of the refractive index , a proxy for the unidentified absorber, from 15,002 pairs of 283 and images taken by the Ultraviolet imager (UVI) onboard Akatsuki. We obtained the distributions of and the unidentified absorber with high spatial resolution every 2 hours for about 6.5 years. We analyzed the local time and latitudinal distributions of and the unidentified absorber. The mixing ratio at the cloud top around altitude depends on latitude and local time, ranging from 80 to , within the range of previous observations. and the unidentified absorber are abundant at low latitudes and decrease toward mid‐latitudes, which is attributed to vertical transport by the Hadley circulation. The abundance is maximized around 14:30 at low latitudes; the distribution is explained by the vertical transport by the semi‐diurnal tide and consistent with the cloud‐top velocity distribution. The local time and latitude dependence of obtained is consistent with a three‐dimensional general circulation model involving photochemical reactions. We also derived the long‐term variations in and the unidentified absorber. increased from 2016 to 2019, showed no clear trend until early 2021, and decreased through 2022. No clear correlation was seen between the unidentified absorber and the mean super rotation speed, although it contributes to the solar heating rate.