The variation in the solar irradiance related for example to the 11-year cycle leads to changes in the photodissociation and photo-ionization of the upper and middle atmosphere. The strength of the particle precipitation is also modified. These phenomena are responsible for cyclic variations in the concentration of neutral constituents such as water vapor, nitric oxide, atomic oxygen and ozone, and of ionic compounds in the E- and D-region. Such effects have been simulated by means of a zonally-averaged two-dimensional model taking into account the latitudinal and seasonal dependences. As an example, the increase of thermospheric nitric oxide with solar activity, the corresponding enhancement of the downward flux of NO reaching the stratosphere during the winter and the impact of this effect on the photoionization will be considered.