The solar UV radiation is one of the most important external agents affecting our atmosphere. The radiation, absorbed in the atmosphere, causes dissociation, excitation and ionization of the atmospheric constituents. As an energy source, the amount of UV radiation defines the temperature structure of the atmosphere. Through dissociation, and consequently following chemical reactions, chemically important neutral minor constituents are produced and destroyed in the atmosphere at all altitudes. The excited states of atmospheric constituents define the balance in the possible chemical reaction channels and produce a multitude of atmospheric emissions. At the Sodankylä Geophysical Observatory we have concentrated on studying the ionization in the upper atmosphere. For this purpose a specific theoretical tool, a detailed ion chemistry model of the lower ionosphere was constructed. With the 55-ion Sodankylä Ion Chemistry model we are able to study the consequences of the ionization processes in the altitude region from 50 to 100 km. During geophysically quiet times the ionization of the lower ionosphere is mainly caused by photoionization of NO, and by galactic cosmic rays . Natural variations in the solar Lyman-a flux at 121.6 nm, capable of ionizing NO, cause significant variations in the ionization balance. We present theoretical estimates of the response of ionosphere to variations in the UV radiation of relevant wavelengths, and discuss the experimental observations of these variations. Ionization processes produced by UV radiation would also offer a possibility to study the long term variations in the upper atmosphere, whether they are of anthropogenic or natural origin. Our research efforts in this direction are shortly described.