Short-term variation in the dayside cloud top structure of Venus: A search for a cause using multispectral images of Akatsuki

Venus is completely shrouded by optically thick clouds of sulfuric acid that are located between ~47 and 70 km. The cloud tops have been investigated through imaging, spectroscopy, and polarimetry in a broad range of wavelengths, from UV to mid-infrared, as well as in-situ measurements. For example, Ignatiev et al. (2009) studied the cloud top altitude from the depth of CO₂ absorption band at 1.6 μm acquired by VIRTIS onboard Venus Express. They found that the cloud tops decreased poleward of 50° and this depression coincided with the eye of the polar vortex. Sato et al. (2020) described the dayside cloud top structure of Venus using the 2.02-μm channel (designed for sensing at a CO₂ absorption band) of the 2-μm camera (IR2) onboard Akatsuki. They showed that the latitudinal structure of the cloud top altitude was symmetric with respect to the equator. The average cloud top altitude was 70.5 km in the equatorial region and showed a gradual decrease of ~2 km by the 45° latitude. It rapidly dropped at latitudes of 50–60° and reached 61 km in latitudes of 70–75°.

We have investigated the solar phase angle dependence of the reflected sunlight in the low-latitude region (30°S–30°N) using the complete set of 2.02-μm images. A total of 374 images taken from December 11, 2015, to October 29, 2016 were selected by carefully excluding those with saturated pixels and/or defect image tiles. In general, the reflected sunlight gets more intense as solar phase angle increases due to the forward scattering of aerosols in the atmosphere. Interestingly, the curve of the reflected sunlight in solar phase angles higher than 90° acquired in Orbit #29–30 was approximately twice more gradual than that acquired in Orbit #11–12. Because these data were taken under the similar geometry, the difference is likely to be caused by some real variations in the cloud top structure. Such solar phase angle dependence of the reflected sunlight or brightness temperature in the same region was also studied using images taken in the same observation period but at other wavelengths: 283 nm and 365 nm from the Ultraviolet imager (UVI), 0.9 μm from the 1-μm camera (IR1), and 10 μm from the Longwave infrared camera (LIR). This multispectral comparison showed that no significant difference in the curve between Orbit #11–12 and #29–30 was detected at the wavelengths other than 2.02 μm. Wavelengths at 283 nm, 365 nm, and 0.9 μm are sensitive to optical thickness of aerosols but insensitive to their vertical distributions; therefore, vertical distributions of Modes 1 and 2 particles characterized by scale height are key parameters to make the difference in the curve of the reflected sunlight discovered from the 2.02-μm images. To support this hypothesis, we selected the data to make two groups: Group 1 (including Orbit #11–12) and Group 2 (including Orbit #29–30). For each group, the observed solar phase angle dependence and the center-to-limb variation of the reflected sunlight in the low-latitude region were used to retrieve cloud top altitude z_c and cloud scale height H by means of radiative transfer calculation. While the best-fit combination for Group 1 is obtained at (z_c, H) = (70.4 km, 5.5 km), that for Group B is found at (z_c, H) = (69.9 km, 3.5 km). The results quantitatively show that the difference can be explained by the 2-km difference of cloud scale height.

In this presentation, we present the solar phase angle dependence of the reflected sunlight or brightness temperature using the five wavelengths of Akatsuki’s instruments. The details of the radiative transfer model and fitting results are shown. Finally, we will discuss whether the best-fit models retrieved from Groups 1 and 2 are compatible with the data at other wavelengths.