# Solar Extreme and Far Ultraviolet Radiation Modeling for Aeronomic Calculations
## 3 Results  
### EUVT Model  
It is assumed that the model will be used primarily for calculating the parameters of the ionosphere and various effects associated with changes in the fluxes of EUV-radiation. Therefore, it will accept a set of lines and spectral intervals included in the EUVAC model [15], which is often used in solving problems of aeronomy. [PERSON] and [PERSON] [13] proposed for calculating the spectrum in the range 10-105 nm, to use the He I \(\lambda=58.4\) nm line as a reference and considered the ratio  
\[\text{R}_{\lambda}=\text{N}_{\lambda}/\text{N}_{\lambda\text{r}}. \tag{2}\]  
where N\({}_{\lambda}\) is the flux of quanta for a line with a wavelength \(\lambda\), N\({}_{\lambda\text{r}}\) is the flux in the reference line, considered as an index of solar activity. Based on the analysis of measurement data on the AE-E satellite and earlier measurements, it was shown [13] that R\({}_{\lambda}\) for lines with ionization temperatures differing even by 2 orders of magnitude changes little in the solar cycle and can be represented in decomposition by activity index N\({}_{\lambda\text{r}}\):  
\[\text{R}_{\lambda}=\text{B}0_{\lambda}+\text{B}1_{\lambda}\text{N}_{\lambda \text{r}}, \tag{3}\]  
where B0\({}_{\lambda}\) and B1\({}_{\lambda}\) are coefficients for each of the lines or spectral intervals with a wavelength \(\lambda\). Equations (2) and (3) correspond to quadratic dependence of N\({}_{\lambda}\) on N\({}_{\lambda\text{r}}\):  
\[\text{N}_{\lambda}=\text{N}_{\lambda\text{r}}\cdot(\text{B}0_{\lambda}+\text{ B}1_{\lambda}\text{N}_{\lambda\text{r}})\]  
According to the data of various experiments, the values of B0\({}_{\lambda}\) and B1\({}_{\lambda}\) were determined for the range 10-105 nm. Comparison of calculation results with measurements shows a good agreement (error \(<10\%\)) in all spectral intervals. It can be assumed that to solve this problem, it is advisable to apply the methodology of [13] using TIMED/SEE data and daily data of the L\(\alpha\) line intensity. The division of the spectrum into separate wavelength intervals is performed in EUV model similarly to that used in the models [13; 15; 16]. When passing to a new reference line of the spectrum, it is additionally necessary to check the property of low variability R\({}_{\lambda}\) for a new index--the flux in the L\(\alpha\) line. The ratio R\({}_{\lambda}\) was considered on the basis of the TIMED/SEE measurement database and using the data on the intensity of the LaC composite line for the period 2002-2017, which included sections of 23-24 cycles of solar activity, corresponding to the minimum of 23-24 cycles,phases of growth and decline of activity and the maximum of the 24 th cycle. It is significant that, starting from 2017, for a number of lines, a sharp increase in R\({}_{\lambda}\) appears with a decrease in the N(L\(\alpha\)T) intensity, which indicates the probable appearance of failures in the operation of the equipment, or a possible revision of the calibration, which was indirectly indicated by [PERSON] et al. [25]. Therefore, the time series used were limited to 2017. Changes in the R\({}_{\lambda}\) values depending on the flux in the L\(\alpha\) line during this period are shown in Figure 2 for several lines. It can be seen that the changes in R\({}_{\lambda}\) are indeed small for almost all levels of solar activity considered. The statistical analysis of the dependences obtained makes it possible to determine the coefficients B0\({}_{\lambda}\) and B1\({}_{\lambda}\) in (2), and to estimate their error also. The resulting set of coefficients, together with Equations (2) and (3), is the model for shortwave radiation. Since the model is based on TIMED satellite data, it will be referred to as EUVT later. The values of the coefficients of the EUVT model are presented in Table 1 for each wavelength or a wavelength interval.