# Solar Extreme and Far Ultraviolet Radiation Modeling for Aeronomic Calculations
## 4 Discussion  
When using the EUVT model to calculate ionization rates and characteristics of the photoelectron spectrum, one should have an idea of its differences from other models of the EUV spectrum. Since the EUVAC model [15] is often used for these purposes, it is advisable to compare the calculations for this model with the corresponding results of the proposed model. Let us first consider the changes in the value of the total radiation flux N\({}_{\rm tot}\) in the region of 10-105 nm, which determines the ionization rate at the heights of the F1 and F2 layers. In Figure 6 the results of calculations of N\({}_{\rm tot}\) daily values are presented for the period 2002-2018. The input parameters for the EUVAC model are the radio flux at 10.7 cm and its average value F10.7A for 81 days. If the data were omitted, the F10.7 values were calculated from the sunspot numbers W (version 2). The Figure 6 shows that the EUVT model gives systematically higher values than the EUVAC model. The ratio of the results is represented by line 3. The average value of the ratio is 1.37. Thus, it can be expected that when using a model based on modern data on EUV radiation, ionization rates in model calculations of the ionosphere will increase by almost -40%, which will accordingly affect the parameters of the ionosphere. Figure 5: Comparison of model calculations with TIMED/SEE data. (**a**) Near the maximum of activity, 21 February 2002, (**b**) Near the minimum of activity, 21 February 2009. Solid line—measurement data, dashed line—model calculation. However, the magnitude of the excess is not the same for different parts of the spectrum. Figure 7 represents the results of model calculations for fluxes in the lines of 30.4 nm (lines 1 and 2) and 102.6 nm (lines 3 and 4). Blue lines correspond to the EUVAC model, and red the EUVT model. It can be seen that in line 30.4 EUVAC is always higher than EUVT (by \(\sim\)30% on average). This fact will affect the modeling of the F-layer. On the contrary, for the 102.6 nm line, calculations by the EUVT model give an excess over EUVAC by an average of -12%. This spectral line (along with the CIII 97.7 nm line) gives the main contribution to the  
Figure 6: Time variations of the total flux of EUV radiation over 23–24 activity cycles according to calculations using the EUVT (**Line 1**) and EUVAC (**Line 2**) models. (**Line 3**)—their ratio r.  
Figure 7: Time variations in the 23–24 activity cycles of individual lines of the EUV region. 1, 2—30.4 nm, 3. 4—102.6 nm. **Blue** lines—EUVAC, **Red**—EUVT. ionization of the ionospheric E layer, i.e., when modeling the ionosphere, both models may give approximately the same results for the E-layer. Thus, when calculating the parameters of the ionosphere at different altitudes, the use of EUVT model for ionospheric calculations can lead to results, that contradict to those obtained using EUVAC. The FUVT model makes it possible to obtain the values of the radiation fluxes in all parts of the spectrum of the considered interval \(115\leq\lambda\leq 242\) nm. In this case, not only the data of current measurements of the input parameter NL\(\alpha\) can be used, but also archival data or its model estimates based on some other data, for example, according to the solar radio emission index F10.7 or according to the data on the critical frequencies of the E layer of the ionosphere [6]. Such estimates are important, for example, when planning long-term space experiments. Figure 8 shows the model spectra calculated for the maximum (solid line) and minimum (dashed line) NL\({}_{\rm{L}\alpha}\) values given in the LISIRD database for the period from 1947 to 2018 (respectively, NL\({}_{\rm{L}\alpha}\) = 3.31 1015 m\({}^{-2}\)s\({}^{-1}\) in 2018 and 7.12 10\({}^{15}\) m\({}^{-2}\)s\({}^{-1}\) in 1947). It can be seen from the figure that when the transition occurs from low solar activity to high activity, the FUV fluxes weakly differ in the spectral region \(\lambda>210\) nm. The changes increase markedly when going to shorter wavelengths.