Reduction in 317–780nm 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 65min 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 (L1), about 1.5millionkm from the Earth. The L1 location permitted seven observations of the Moon's shadow on the Earth for about 3h 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 (λi=388, 443, 551, 680, and 780nm) 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 (λi=317.5, 325, 340, 688, and 764nm) that are partially absorbed in the atmosphere give consistent reductions compared to the non-absorbed channels. This indicates that cloud reflectivities dominate the 317.5–780nm 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 781nm) reflected from the sunlit Earth towards L1 for two sets of observations on 21?August 2017, while the shadow was in the vicinity of Casper, Wyoming (42.8666°N, 106.3131°W; centered on 17:44:50UTC), and Columbia, Missouri (38.9517°N, 92.3341°W; centered on 18:14:50UTC). 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 443nm compared to 9% (Casper) and 8% (Columbia) during the eclipse). Also measured was the ratio REN(λi) 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 REN(443nm) was smaller for Columbia (169) than for Casper (935), because Columbia had more cloud cover than Casper. REN(λi) 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 REN(340nm)=475, REN(388nm)=3500, REN(443nm)=935, REN(551nm)=5455, REN(680nm)= 220, and REN(780nm)=395. Some of the variability is caused by changing cloud amounts within the moving region of totality during the 2.7min needed to measure all 10?wavelength channels.