  We present new measurements of the vertical density profile of the Earth's
atmosphere at altitudes between 70 and 200 km, based on Earth occultations of
the Crab Nebula observed with the X-ray Imaging Spectrometer onboard Suzaku and
the Hard X-ray Imager onboard Hitomi. X-ray spectral variation due to the
atmospheric absorption is used to derive tangential column densities of the
absorbing species, i.e., N and O including atoms and molecules, along the line
of sight. The tangential column densities are then inverted to obtain the
atmospheric number density. The data from 219 occultation scans at low
latitudes in both hemispheres from September 15, 2005 to March 26, 2016 are
analyzed to generate a single, highly-averaged (in both space and time)
vertical density profile. The density profile is in good agreement with the
NRLMSISE-00 model, except for the altitude range of 70-110 km, where the
measured density is about 50% smaller than the model. Such a deviation is
consistent with the recent measurement with the SABER aboard the TIMED
satellite (Cheng et al. 2020). Given that the NRLMSISE-00 model was constructed
some time ago, the density decline could be due to the radiative
cooling/contracting of the upper atmosphere as a result of greenhouse warming
in the troposphere. However, we cannot rule out a possibility that the NRL
model is simply imperfect in this region. We also present future prospects for
the upcoming Japan-US X-ray astronomy satellite, XRISM, which will allow us to
measure atmospheric composition with unprecedented spectral resolution of dE ~
5 eV in 0.3-12 keV.
