  We report the results of detailed investigation of the Earth's transmission
spectra during the lunar eclipse on UT 2011 December 10. The spectra were taken
by using the High Dispersion Spectrograph (HDS) mounted on the Subaru 8.2 m
telescope with unprecedented resolutions both in time and wavelength (300 s
exposure time in umbra and 160,000 spectral resolution, respectively). In our
penumbra and umbra data, we detected the individual absorption lines of O2 and
H2O in transmission spectra and found that it became deeper as the eclipse
became deeper. It indicates that the sunlight reaching the Moon passed through
lower layers in the Earth's atmosphere with time because we monitored a given
point on the Moon during the full eclipse duration. From the comparison between
the observed and theoretically constructed transmission spectra, the lowest
altitude at which the sunlight actually passed through the atmosphere is
estimated to be about 10 km from the ground, which suggests the existence of
sunlight blocking clouds below that altitude. Our result would be a test case
for future investigations of atmospheric structure of Earth-like exoplanets via
transmission spectroscopy including the refraction effect of the planetary
atmosphere.
