Reduction in 317–780?nm radiance reflected from the sunlit Earth during the eclipse of 21?August 2017

摘要

Ten wavelength channels of calibrated radiance image data from the sunlit Earth are obtained every 65?min during Northern Hemisphere summer from the EPIC (Earth Polychromatic Imaging Camera) instrument on the DSCOVR (Deep Space Climate Observatory) satellite located near the Earth–Sun Lagrange?1 point (L sub1/sub ), about 1.5?million?km from the Earth. The L sub1/sub location permitted seven observations of the Moon's shadow on the Earth for about 3?h during the 21?August 2017 eclipse. Two of the observations were timed to coincide with totality over Casper, Wyoming, and Columbia, Missouri. Since the solar irradiances within five channels ( λsubi/sub=388 , 443, 551, 680, and 780?nm) are not strongly absorbed in the atmosphere, they can be used for characterizing the eclipse reduction in reflected radiances for the Earth's sunlit face containing the eclipse shadow. Five channels ( λsubi/sub=317.5 , 325, 340, 688, and 764?nm) that are partially absorbed in the atmosphere give consistent reductions compared to the non-absorbed channels. This indicates that cloud reflectivities dominate the 317.5–780?nm radiances reflected back to space from the sunlit Earth's disk with a significant contribution from Rayleigh scattering for the shorter wavelengths. An estimated reduction of 10?% was obtained for spectrally integrated radiance (387 to 781?nm) reflected from the sunlit Earth towards L sub1/sub for two sets of observations on 21?August 2017, while the shadow was in the vicinity of Casper, Wyoming (42.8666 sup°/sup N, 106.3131 sup°/sup W; centered on 17:44:50?UTC), and Columbia, Missouri (38.9517 sup°/sup N, 92.3341 sup°/sup W; centered on 18:14:50?UTC). In contrast, when non-eclipse days (20 and 23?August) are compared for each wavelength channel, the change in reflected light is much smaller (less than 1?% for 443?nm compared to 9?% (Casper) and 8?% (Columbia) during the eclipse). Also measured was the ratio RsubEN/sub(λsubi/sub) of reflected radiance on adjacent non-eclipse days divided by radiances centered in the eclipse totality region with the same geometry for all 10?wavelength channels. The measured RsubEN/sub (443?nm) was smaller for Columbia (169) than for Casper (935), because Columbia had more cloud cover than Casper. RsubEN/sub(λsubi/sub) forms a useful test of a 3-D radiative transfer models for an eclipse in the presence of optically thin clouds. Specific values measured at Casper with thin clouds are RsubEN/sub (340?nm) = 475, RsubEN/sub (388?nm) = 3500, RsubEN/sub (443?nm) = 935, RsubEN/sub (551?nm) = 5455, RsubEN/sub (680?nm) = 220, and RsubEN/sub (780?nm) = 395. Some of the variability is caused by changing cloud amounts within the moving region of totality during the 2.7?min needed to measure all 10?wavelength channels.