Using polarization data from SPICAV IR to reanalyze the bimodal distribution and microphysical properties of Venus' upper haze

• Quantifies how cloud/haze microphysics and size modes shape Venusian polarization. • Constrains r eff , τ and γ via sensitivity tests and multi-orbit fits, supporting bimodality. • Assesses high-index particle effects, showing polarization can help identify them. We investigate the bimodal particle size distribution and microphysical properties of Venus’ upper haze using SPICAV-IR polarization observations and radiative-transfer modeling. The observed linear polarization is mainly controlled by single scattering in the upper cloud and haze, with submicron particles showing Rayleigh-like behavior near a phase angle of ∼80°. Sensitivity analyses constrain key parameters—including effective radius, optical thickness, and number-density ratio—and are used to construct a lookup-table-based retrieval framework. Compared with unimodal models, the bimodal representation consistently provides a better fit to the observations, with total R 2 > 0.8 at multiple near-IR wavelengths. Multi-parameter retrievals indicate mean upper-haze optical depths of τ ∼ 0.10 in the northern polar region and 0.15 ± 0.02 at low latitudes. Retrieved particle sizes show clear latitudinal variability, with the effective radius of Mode 2a decreasing from ∼0.91 μm in the southern hemisphere to ∼0.75 μm toward the poles, while Mode 1a remains submicron and the number-density ratio γexhibits no robust latitudinal trend. The smaller polar particle size accounts for the enhanced polar polarization, and sensitivity tests suggest the possible presence of high-refractive-index particles near the cloud top, accompanied by a slight shift in the glory phase angle. These results provide new quantitative constraints on the microphysics and polarization behavior of Venus’ upper haze.