Long-Term Density Trend in the Mesosphere and Lower Thermosphere from Occultations of the Crab Nebula with X-Ray Astronomy Satellites

TL;DR

This study analyzes 28 years of X-ray occultation data from multiple satellites to identify long-term density trends in Earth's upper atmosphere, revealing a significant decline near 110 km possibly linked to cooling effects.

Contribution

It provides the first observational evidence of a strong density decline at 110 km, supporting numerical predictions of cooling due to water vapor and ozone.

Findings

Approximately -5%/decade density decline at all altitudes studied.

An exceptionally high decline of about -12%/decade near 110 km.

No significant differences among seasonal and local time trends.

Abstract

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 degrees. With this constraint, local times are automatically limited either around noon or midnight. We obtain four sets (two seasons times 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.