Estimation of Adiabatic Cooling and Warming in the Mesosphere and Lower Thermosphere

In this paper, we estimate the adiabatic cooling and warming in the MLT utilizing the SABER CO₂ VMR displacement from the global mean. This confirms that the summer mesopause temperature is largely controlled by adiabatic cooling instead of any absorptive heating or chemical heating. Because the adiabatic cooling is dynamically driven by waves from below, the summer polar mesopause is mostly sensitive to the changes in the stratosphere and mesosphere, for example, Sudden Stratospheric Warmings (SSWs) and polar vortex. And it well explains that the Aeronomy of Ice In the Mesosphere (AIM) satellite did not observe solar cycle responses in PMCs over the latest solar cycles. Unlike UV radiative heating in the upper atmosphere, dynamical cooling and mesosphere dynamics may have a complex relationship with the solar cycle. The paper also reveals a previously overlooked layer of adiabatic warming in summer and cooling in winter in the lower thermosphere due to downwelling and upwelling. Because this process is embedded in the thermosphere where mean temperature rises sharply driven by diffusive heating (or heat conduct from the upper thermosphere), it is not obvious without removing the global mean temperature. The mesosphere is the opposite, being lacking of strong heating sources. The heating layer (~100 K) in the summer lower thermosphere is substantial. Auroral heating also occurs in the magnetic polar lower thermosphere. How the adiabatic heating and cooling in the polar lower thermosphere interacts with auroral heating and the Joule heating driven adiabatic heating and cooling during geomagnetic active times warrants further investigations.