Spatiotemporal Variability of Venus's Cloud-Top Absorbers: Eight Years of Akatsuki Observations

Ultraviolet (UV) absorption at Venus’s cloud tops drives atmospheric circulation, yet long-term variability remains difficult to disentangle from observations. We analyzed eight years (2016-2024) of Akatsuki Ultraviolet Imager observations at 283 and 365 nm to investigate the absorbers’ spatiotemporal variability. Co-registered 1deg x 1deg grid reveals persistent latitudinal stratification, with systematically lower 283/365 nm UV color ratios in the equatorial region (15deg S - 15deg N) than at mid-latitudes (20deg - 45deg). Since solar phase angle strongly biases these ratios and cycles near the 224.7-day Venusian orbit, we decoupled viewing geometry from temporal evolution using a joint harmonic regression. This simultaneous fit isolates a statistically significant, intrinsic atmospheric oscillation at 217.1 Earth days ( delta BIC =63.1 vs. the orbital phase). Distinct from Venus’s orbital phase, this 217.1-day cycle reveals a multi-month physical rhythm in the equatorial cloud deck, likely driven by a radiative-convective recharge thermostat or low-frequency wave-mean flow interactions.