New Measurement of the Vertical Atmospheric Density Profile from Occultations of the Crab Nebula with X-Ray Astronomy Satellites Suzaku and Hitomi

TL;DR

This study uses X-ray occultation data from Suzaku and Hitomi satellites to measure Earth's atmospheric density profile between 70-200 km, revealing deviations from existing models and suggesting possible climate-related atmospheric changes.

Contribution

First to derive a high-precision vertical density profile of Earth's atmosphere using X-ray occultations from space-based observatories.

Findings

Measured density is about 50% smaller than the NRLMSISE-00 model between 70-110 km.

Data agrees with recent SABER measurements, indicating potential climate effects.

Future X-ray missions like XRISM will enhance atmospheric composition measurements.

Abstract

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.