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Current satellite radiation products from polar-orbiting sensors like MODIS, SGLI, VIIRS, and OCI provide only daily averaged quantities, missing critical diurnal variations. These fluctuations in light energy, ranging from none to ample throughout the day, significantly influence photosynthetic communities, impacting carbon export, nutrient cycling, and ecosystem functions. Incorporating hourly changes in phytoplankton light absorption and irradiance, as shown with GOCI data, has enhanced estimates of net primary productivity. However, the global oceans cannot be fully observed from a single geostationary platform, especially at high latitudes. EPIC, positioned at the first Lagrange point (L1) about 1.5 million km from Earth, offers a unique advantage by simultaneously capturing the entire sunlit ocean with high temporal resolution, enabling detailed observation of evolving systems and diurnal phenomena. Unlike polar orbiters, EPIC provides better coverage at low and middle latitudes, effectively mitigates Sun glint, and offers adequate views of polar regions. Utilizing EPIC data, we estimate hourly photosynthetically available radiation (PAR) over the global ocean. The algorithm is developed and validated through radiative transfer simulations under realistic conditions. Comparisons with geostationary AHI and GOCI data show strong performance. The diurnal PAR estimates, when combined with other ocean color products, are expected to advance studies of aquatic photosynthesis and biogeochemistry.
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