Solar influence on mesospheric water vapor with impact on NLCs

On the basis of our global 3d-model of the dynamics and chemistry of the middle atmosphere COMMA-IAP we investigate the influence of the variation of solar activity on the composition of the mesosphere/lower thermosphere. In the mesosphere, unlike the thermosphere, the almost only marked influence results from the change of the Lyman-α irradiance varying maximum by a factor about two over a solar cycle. The Lyman-α radiation has the strongest impact on the water vapor distribution above the upper mesosphere. Water vapor is the key compound of the entire mesosphere determining all other chemically active species due to the hydrogen radical production from this source constituent. As a result, the thermally active species feedback to the dynamics. Water vapor is also the constituent for the formation of NLCs and PMSEs in high latitude in summer. Above all, we consider the variation of the water vapor mixing ratio. Special attention is given to the chemical composition of the NLC region at high latitudes. As expected we find an increasing relative response to the water vapor mixing ratio with increasing height. Due to the strongly increasing absorption of the Lyman-α radiation below about 70 km, the variations become increasingly smaller and are irrelevant and non-definable at 60 km. The variations are normally in anti-phase to the solar activity, usually without a distinct phase delay. Other species such as ozone are in phase with the solar cycle. The OH radical mixing ratio varies more irregularly, but the coarse variation agrees with the photolysis of water vapor with the highest values around solar maximum.