Solar Cycle Response of CO₂ Over the Austral Winter Mesosphere and Lower Thermosphere Region

This work uses Sounding of the Atmosphere using Broadband Emission Radiometry CO₂ data from 2002 to 2015 and Specified Dynamics-Whole Atmosphere Community Climate Model (SD-WACCM) outputs from 1979 to 2014 to show, for the first time, the solar cycle response of CO₂ in the Austral winter mesosphere and lower thermosphere region. Both Sounding of the Atmosphere using Broadband Emission Radiometry and SD-WACCM show that CO₂ experiences a decrease during solar maximum throughout the Austral winter mesosphere and lower thermosphere region. This work highlights the regions where CO₂ experiences its strongest and weakest solar cycle responses as modeled by SD-WACCM. The region with the strongest solar cycle response experiences around 5% reduction in CO₂ between solar maximum and solar minimum. The region with weakest solar cycle response experiences less than 1% reduction in CO₂ between solar maximum and solar minimum. It is shown that the region of the strongest CO₂ response is driven by photodissociation, downwelling, and reduced eddy diffusion. On the other hand, the region of the weakest CO₂ response is driven by the opposing effects of photodissociation and enhanced eddy diffusion. This is the first work to show that the solar cycle could affect the Austral winter lower thermosphere circulation and eddy diffusion processes. These anomalies in the lower thermospheric circulation and eddy diffusion are found to be related to the solar cycle response in the Austral winter mesosphere wave-mean flow dynamics. This work therefore concludes that the solar cycle affects lower thermospheric CO₂ via modulations of the lower thermospheric circulation and eddy diffusion processes.