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

Abstract

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 ₁), about 1.5 million km from the Earth. The L ₁ 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 (λi = 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 (λi = 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 ₁ 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 17:44:50 UTC), and Columbia, Missouri (38.9517◦ N, 92.3341◦ 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 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(443 nm) 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(340 nm) = 475, Rₑₙ(388 nm) = 3500, Rₑₙ(443 nm) = 935, Rₑₙ(551 nm) = 5455, Rₑₙ(680 nm) = 220, and Rₑₙ(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.