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

[PPS04-15] A follow-up analysis of the Venusian cloud top altitude using Akatsuki IR2 images

★Invited Papers

*Takao M. Sato¹, Takehiko Satoh^2,3 (1.Hokkaido Information University, 2.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3.The Graduate University for Adcanced Studies, SOKENDAI)

Keywords:Venus, cloud top structure, infrared, Akatsuki, radiative transfer

Sato et al. (2020) described the dayside cloud top structure of Venus as retrieved from 93 images acquired at a wide variety of solar phase angles (0-120 deg) using the 2.02-μm channel of the 2-μm camera (IR2) onboard the Venus orbiter, Akatsuki, from April 4 to May 25, 2016. Since the 2.02-μm channel is located in a CO₂ absorption band, the sunlight reflected from Venus allowed us to determine the cloud top altitude corresponding to a unit aerosol optical depth at 2.02 μm. First, the observed solar phase angle dependence and the center-to-limb variation of the reflected sunlight in the region equatorward of 30 deg were used to construct a spatially averaged cloud top structure characterized by cloud top altitude z_c, Mode 2 modal radius r_g,2, and cloud scale height H, which were 70.4 km, 1.06 μm, and 5.3 km, respectively. Second, cloud top altitudes at individual locations were retrieved on a pixel-by-pixel basis with an assumption that r_g,2 and H were uniform for the entire planet. 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 deg latitude. It rapidly dropped at latitudes of 50-60 deg and reached 61 km in latitudes of 70-75 deg. The average cloud top altitude in the region equatorward of 30 deg showed negligible local time dependence, with changes up to 1 km at most.
We applied the method described above to all available images taken by IR2 (from December 11, 2015, to October 30, 2016) to better understand the latitudinal and local time variations in cloud top altitude of Venus. After the publication of Sato et al. (2020), a method for identifying saturated pixels of IR2 image was updated. In addition, the spectroscopic line parameters used for radiative transfer model were also replaced by the latest HITRAN2020 database. In this presentation, we present initial results of this follow-up analysis regarding the cloud top altitude of Venus using IR2 images.