  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.
