How the Venusian Atmospheric Dynamics Shapes Cloud-Top Sulfur Dioxide Derived from Akatsuki/UVI

Introduction

The 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.

Methods

We 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.

Results

A 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.