Estimation of adiabatic cooling and warming in the mesosphere and lower thermosphere

The adiabatic cooling and warming in the mesosphere and lower thermosphere (MLT) are estimated utilizing the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) measured CO₂ volume mixing ratio (VMR) vertical displacement from the global mean CO₂ VMR. This work confirms that the summer mesopause temperature is largely controlled by adiabatic cooling instead of any absorptive heating or chemical heating. The paper also reveals a previously overlooked layer of adiabatic warming in summer and adiabatic cooling in winter in the lower thermosphere, being driven by downwelling and upwelling associated with the winter-to-summer circulation. Since this adiabatic warming/cooling process is embedded in the thermosphere where the mean temperature rises sharply, it is not as distinct without removing the global mean temperature. The mesospheric temperature is the opposite, being lacking of strong heating sources. The heating anomaly (∼100 K) in the summer lower thermosphere is substantial. Because auroral heating also occurs in the geomagnetic polar lower thermosphere, the interaction between the adiabatic warming/cooling in the lower thermosphere and auroral heating should be considered in future studies.