Solar activity control of ionospheric and thermospheric processes.

Ionospheric electron content observations carried out in Neustrelitz (53.3°N, 13.1°E) and Havana, Cuba (23.1°N, 82.5°W) are analyzed with respect to their solar activity dependence. Cross correlation studies with the solar radio flux at 10.7 cm wavelength indicate a response time of the ionosphere to the 27-day solar radiation cycle of the order of 1-2 days. It is assumed that this time shift is caused by solar radiation control of the atomic oxygen concentration in the thermosphere which follows the change of the solar radiation due to photodissociation of molecular oxygen, but with a time lag of several days. This hypothesis is discussed using a one-dimensional numerical model which includes photochemistry and diffusion processes. To a first approximation, temperature changes have been neglected and use is made of a constant temperature profile according to CIRA 72. The results demonstrate a time lag of the atomic oxygen concentration variation of 2 days with respect to the solar radiation variation. The 27-day run of the model indicates changes in the concentration of atomic oxygen by about 7% at 180 km height.