# Solar Extreme and Far Ultraviolet Radiation Modeling for Aeronomic Calculations
## 1 Introduction  
Extreme and far ultraviolet radiation is the most significant of the entire solar spectrum for the formation and composition of the ionosphere and upper atmosphere. The impact of radiation on the upper atmosphere leads to a number of photochemical processes, which are mainly made up of ionization and dissociation of atmospheric gas molecules. The origination of atomic oxygen is due to the dissociation of oxygen molecules under the influence of ultraviolet radiation and transfer processes. The dissociation threshold corresponds to the radiation wavelength \(\lambda\) = 242 nm [1]. At altitudes of more than \(\sim\)120 km, the concentrations of atomic and molecular oxygen become equal, and at altitudes of \(\sim\)180 km, the concentration of atomic oxygen also exceeds the concentration of nitrogen molecules, and atomic oxygen becomes the main component of the upper atmosphere (e.g., [2]). At wavelengths shorter than 104 nm, atmospheric gases are ionized (see, for example, [2]). For this part of the spectrum, the production of oxygen atoms can be neglected. In addition to dissociation, absorption in the atmosphere by FUV leads to its heating (see, for example, [3]). The thermosphere and ionosphere are influenced by the joint efforts of solar radiation, geomagnetic activity and the lower atmosphere forcing. All these terms play important roles in the distribution of the thermosphere and ionosphere composition. The importance of taking into account variations in the flux of UV radiation with the level of activity in the study of the upper atmosphere was emphasized by [PERSON] et al. [4]: if during the transition from the minimum solar activity to the maximum, the temperature changes at an altitude of \(\sim\)10 km is \(\sim\)0.1 K, and \(\sim\)1 K by 50 km, then at altitudes of \(\sim\)500 km, in the thermosphere, the changes exceed 400 K. Thus, to model the main processes of the formation of the upper atmosphere and ionosphere, knowledge of solar radiation fluxes in the region of 10-242 nm is required. Ultimately, long-term changes in the upper atmosphereare due to variations in this range. Modeling the spectrum of solar ultraviolet radiation is an important integral task of a number of other problems, such as the effect of radiation on the surface of spacecraft, problems of space materials science. or changes in atmospheric ozone (e.g., [4]). Simulation of FUV fluxes under various heliophysical conditions is essential for solving some practical problems arising in the development of space technology. In particular, it is necessary to take into account the factors of influence that lead to the degradation of various components of spacecraft, and are determined, along with the fluxes of ultraviolet radiation, for example, photoelectrons, fluxes of oxygen ions O\({}^{+}\), and other charged particles of low energy, the characteristics of which are due to the state of the upper atmosphere. The actual issue is the problem of \"atmospheric subsidence\" i.e., the existence of a systematic decrease in the density of the upper atmosphere at fixed altitudes (see, for example, [5]). One of the possible mechanisms of such a change is a decrease in the intensity of heating of the atmosphere and the rate of dissociation of molecular oxygen due to a decrease in the FUV flux. Models of the solar extreme ultraviolet (EUV) and far ultraviolet (FUV) spectra are used to quickly determine the characteristics of the near-Earth space environment, subject to adaptation to data from ground-based and space-based monitoring facilities. The radiation model makes it possible to trace long-term variations in the FUV using archived data on input parameters for a long time exceeding the duration of the epoch of satellite observations, for example, using ionospheric data [6].