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. This work presents a connection between the density variationwithin the mesosphere and lower thermosphere (MLT) and changes in theintensity of solar radiation. On a seasonal timescale, these changes takeplace due to the revolution of the Earth around the Sun. While the Earth,during the northern-hemispheric (NH) winter, is closer to the Sun, the uppermesosphere expands due to an increased radiation intensity, which results inchanges in density at these heights. These density variations, i.e., avertical redistribution of atmospheric mass, have an effect on the rotationrate of Earth's upper atmosphere owing to angular momentum conservation. Inorder to test this effect, we applied a theoretical model, which shows adecrease in the atmospheric rotation speed of about∼4 m s−1 at a latitude of 45∘in the case of a density change of 1 % between 70 and 100 km. To supportthis statement, we compare the wind variability obtained from meteor radar(MR) and Microwave Limb Sounder (MLS) satellite observations with fluctuations in the length of a day(LOD). Changes in the LOD on timescales of a year and less are primarilydriven by tropospheric large-scale geophysical processes and their impact onthe Earth's rotation. A global increase in lower-atmosphericeastward-directed winds leads, due to friction with the Earth's surface, to anacceleration of the Earth's rotation by up to a few milliseconds perrotation. The LOD shows an increase during northern winter and decreasesduring summer, which corresponds to changes in the MLT density due to theEarth–Sun movement. Within the MLT the mean zonal wind shows similarfluctuations to the LOD on annual scales as well as longer time series, whichare connected to the seasonal wind regime as well as to density changesexcited by variations in the solar radiation. A direct correlation betweenthe local measured winds and the LOD on shorter timescales cannot clearly beidentified, due to stronger influences of other natural oscillations on thewind. Further, we show that, even after removing the seasonal and 11-yearsolar cycle variations, the mean zonal wind and the LOD are connected byanalyzing long-term tendencies for the years 2005–2016.

Connection between the length of day and wind measurements in the mesosphere and lower thermosphere at mid- and high latitudes