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 (L1), about 1.5 million km from the Earth. The L1 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
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 17:44:50 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.