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

New Measurement of the Vertical Atmospheric Density Profile From Occultations of the Crab Nebula With X‐Ray Astronomy Satellites Suzaku and Hitomi