# Solar Extreme and Far Ultraviolet Radiation Modeling for Aeronomic Calculations
## 1 Introduction
Despite the urgency of the problem of creating a sufficiently accurate and easily used model of FUV radiation, which adequately describes both the magnitudes of the radiation fluxes and their variations for different time scales, there are still no models that reliably reproduce the radiation fluxes in this range at different levels of solar activity. There is a reference spectrum for the 2008 minimum of activity [7], methods for calculating the spectrum for wavelengths from 200 nm were proposed [8; 9; 10]. The NRLSSI [9] and SATIRE [11] models, in principle, satisfy the above requirements. However, it is difficult to use them. This is because it requires simultaneous observation of sunspots, flare areas, and Mg-index. In addition, [PERSON] et al. noted [12] that the NRLSSI model gives significantly underestimated results in comparison with reliable measurements onboard SORCE mission. When developing EUV models [13; 14; 15; 16], relatively small datasets were used, often from different sources, which led to an additional loss of accuracy. It can be expected that the existing models used in the tasks of monitoring the geophysical environment can be noticeably improved due to the emergence of new data over the past decades. The direction of improving predictive models is to use data from monitoring the Sun and the interplanetary medium by ground-based observatories and space vehicles. Along with the change in the volume of the initial data, the set of control parameters of the models has also changed significantly recently. Currently, in the EUV models, along with ground-based measurements (as a rule, radio fluxes at a wavelength of 10.7 cm, F10.7, as, for example, in the EUVAC model [15]), the results of observations on spacecraft are used. These are, for example, models EUV-81 [16], FISM [17; 18], and SOLAR2000 [19], with several intensities of spectral lines as control parameters, or the magnesium index is used, with the ratio of intensities in the core and wings of the MgII line at wavelengths near 280 nm [20]. The possibility of combining spectral measurements in the FUV region with the MgII index [21], or the \"color index\" [22], which changes in time like the MgII index, is being considered. It was noted [23] that different measurements of the MgII index contradict each other and, like other indices, require correction. The flux in the 58.4 nm He II line is used as an index of solar activity for calculating the spectrum of ultraviolet radiation [13; 14]. Thus, it turns out that in order to use modern models of EUV radiation for monitoring space weather, it is necessary to measure the intensity in several lines or parts of the spectrum in the EUV region with a spectral resolution of no worse than 0.1 nm. At present, it is difficult to hope for the implementation of such measurements in the monitoring mode due to their technical complexity and high cost. As an alternative to spectral measurements, it is possible to propose monitoring the EUV radiation by simpler and cheaper means, i.e., narrow-band photometers, which select a narrow part of the spectrum in the EUV region. The photon flux in the 121.6 nm region (L\(\alpha\), Lyman-alpha hydrogen line) seems to be the most suitable for this purpose. Accordingly, using existing models, they must be modified to accommodate L\(\alpha\) measurements as input parameters. The EUV model [13; 14] is based on the property of the spectrum in the EUV region, that the ratio R\({}_{\lambda}\) of the intensities of most lines and spectral intervals to the intensity of the HeI reference line with a wavelength of \(\lambda\) = 58.4 nm varies slightly with the solar activity level (except for a few coronal lines).