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 $\sim$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 $\Delta E \sim 5$\,eV in 0.3–12\,keV.

We present long-term density trends of the Earth’s upper atmosphere at altitudes between 71 and 116 km, based on atmospheric occultations of the Crab Nebula observed with X-ray astronomy satellites, ASCA, RXTE, Suzaku, NuSTAR, and Hitomi. The combination of the five satellites provides a time period of 28 yr from 1994 to 2022. To suppress seasonal and latitudinal variations, we concentrate on the data taken in autumn (49 < doy < 111) and spring (235 < doy < 297) in the northern hemisphere with latitudes of 0◦–40◦. With this constraint, local times are automatically limited either around noon or midnight. We obtain four sets (two seasons × two local times) of density trends at each altitude layer. We take into account variations due to a linear trend and the 11-yr solar cycle using linear regression techniques. Because we do not see significant differences among the four trends, we combine them to provide a single vertical profile of trend slopes. We find a negative density trend of roughly −5%/decade at every altitude. This is in reasonable agreement with inferences from settling rate of the upper atmosphere. In the 100–110 km altitude, we found an exceptionally high density decline of about −12%/decade. This peak may be the first observational evidence for strong cooling due to water vapor and ozone near 110 km, which was first identified in a numerical simulation by Akmaev et al. (2006). Further observations and numerical simulations with suitable input parameters are needed to establish this feature.

We developed a novel ice-core laser melting sampler (LMS) to measure the stable water isotope ratios (δ18O and δD) as temperature proxies at ultra-high depth resolutions. In this LMS system, a 2-mm diameter movable evacuation nozzle holds an optical fiber through which a laser beam irradiates the ice core. The movable nozzle intrudes into the ice core, the laser radiation meanwhile melting the ice cylindrically, and the meltwater is pumped away simultaneously through the same nozzle and transferred to a vial for analysis. To avoid isotopic fractionation of the ice-core components by vaporization, the laser power is adjusted to ensure that the temperature of the meltwater is always kept well below its boiling point. Internal contamination and cross-contamination were both found to be negligible using this LMS. A segment of a Dome Fuji shallow ice core (Antarctica), using the LMS, was then demonstrated to have been discretely sampled with a depth-resolution as small as 3 mm: subsequent measurements of δ18O and δD were reasonably consistent with results obtained by hand segmentation. The LMS will thus enable us to seek the past temperature variations that may appear even in sub-centimeter resolutions in ice cores.