Feasibility study of upper atmosphere density measurement on the ISS by observations of the CXB transmitted through the Earth rim

TL;DR

This study explores the feasibility of measuring upper atmosphere density at around 100 km altitude using cosmic X-ray background observations from the ISS, developing an instrument called SUIM for continuous atmospheric occultation measurements.

Contribution

The paper presents a feasibility analysis for SUIM, an instrument designed to measure atmospheric density via CXB absorption, including estimations of CXB statistics and background noise levels.

Findings

CXB statistics are sufficient for 15 km altitude resolution.

NXB dominates the observed spectra, comprising about 80% of the total.

Estimated NXB levels are comparable to those of the Hitomi satellite's soft X-ray Imager.

Abstract

Measurements of the upper atmosphere at ~100 km are important to investigate climate change, space weather forecasting, and the interaction between the Sun and the Earth. Atmospheric occultations of cosmic X-ray sources are an effective technique to measure the neutral density in the upper atmosphere. We are developing the instrument SUIM dedicated to continuous observations of atmospheric occultations. SUIM will be mounted on a platform on the exterior of the International Space Station for six months and pointed at the Earth's rim to observe atmospheric absorption of the cosmic X-ray background (CXB). In this paper, we conducted a feasibility study of SUIM by estimating the CXB statistics and the fraction of the non-X-ray background (NXB) in the observed data. The estimated CXB statistics are enough to evaluate the atmospheric absorption of CXB for every 15 km of altitude. On the other hand, the NXB will be dominant in the X-ray spectra of SUIM. Assuming that the NXB per detection area of SUIM is comparable to that of the soft X-ray Imager onboard Hitomi, the NXB level will be much higher than the CXB one and account for ~80% of the total SUIM spectra.