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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 years 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‐year 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, https://doi.org/10.1016/j.jastp.2006.03.008). Further observations and numerical simulations with suitable input parameters are needed to establish this feature. Plain Language Summary: Numerical simulations have shown that, while an increase of greenhouse gases such as CO2 in the atmosphere causes heating of the troposphere (near surface), it causes cooling of the middle and upper atmosphere, which is the so‐called "greenhouse cooling." The greenhouse cooling should result in atmospheric contraction and consequently a temporal density decrease at a fixed height. However, observational evidence for the density decrease has been scarce in the mesosphere and lower thermosphere (MLT: 80–110 km), owing to difficulty in measuring the density in this region. Here, we present the first direct measurements of long‐term variations for combined N and O atom number density in the MLT, based on atmospheric occultations of the Crab Nebula observed with X‐ray astronomy satellites. The combination of five X‐ray astronomy satellites, ASCA, RXTE, Suzaku, NuSTAR, and Hitomi, allows us to explore density trends for a long period from 1994 to 2022. We take into account variations due to a temporal linear trend and the 11‐year solar cycle, using linear regression techniques. As a result, we find a negative density trend of roughly −5%/decade at every altitude, with a local minimum of −12%/decade near 105 km. This is in reasonable agreement with the state‐of‐the‐art numerical simulations. Key Points: Time series of combined O and N densities are measured in the MLT, based on atmospheric occultations of the Crab Nebula using X‐ray astronomy satellitesThe density is decreasing everywhere, with a local minimum of −12%/decade near 105 kmThe local minimum in density trends may be due to strong cooling by water vapor and ozone, as was first predicted by Akmaev et al. (2006, https://doi.org/10.1016/j.jastp.2006.03.008) [ABSTRACT FROM AUTHOR]

The Crab nebula is regarded as one of the most important “cosmic laboratories” in astrophysics, which has made a bigger impact on the development of astronomy than any other single object beyond the solar system. The most intriguing recent result is the completely unexpected discovery of a peculiar “jet-torus” structure in the inner part of the nebula. Similar structures were found later in other Crab-like nebulae. This discovery clearly indicates significant anisotropy of the wind from the Crab pulsar which has been ignored so far in simplified theoretical models of the nebula. Fortunately, the impressive progress in computational relativistic magnetohydrodynamics in recent years has made possible to study the Crab nebula without making such a drastic simplification of the problem. In this paper we present the results of the first study of such kind. They provide a likely explanation of the jet-torus pattern and show that the flow in the nebula is much more complex than it has been widely believed. [ABSTRACT FROM AUTHOR]