## 4 Discussion and Conclusions  
In this study, the estimated CRII rate and ambient dose equivalent rate distributions all over the globe were presented for the most common commercial flight levels (FL310, FL350 and FL390) during the recent two solar cycles 23 and 24 (1996-2019). For the calculation of the CRII and the dH\({}^{*}\)(10)/dt, the CRAC:CRII model of Oulu University [12,13] and the DYNASTIMA/DYASTIMA-R software [39,40] were used, respectively. The distribution of both physical quantities was initially depicted in the maps, where the values during the minima and maxima of solar cycles 23 and 24 (for FL390) were illustrated for the entire Earth and all the cut-off rigidities (0-17 GV). The maximum values were observed during the solar minima and at polar regions (approximately 7 \(\mu\)Sv/h at FL390), while the minimum values during the solar maxima and at the equatorial regions were approximately 1.2 \(\mu\)Sv/h at FL390, due to the anticipated anticorrelation of the cosmic ray intensity with the solar activity, as well as due to the shielding of the geomagnetic field. Furthermore, a comparison between solar activity and ionization can be presented. Using the average sunspot number (ASN) as a measure of solar activity (8 for 1996, 180.3 for 2001, 8.4 for 2009 and 114 for 2014), we can get the following ratios: \(\mathrm{ASN}_{2001}/\mathrm{ASN}_{2014}=1.58\) and \(\mathrm{ASN}_{1996}/\mathrm{ASN}_{2009}=0.95\). By calculating the correspondingrations for the maximum values of CRII at FL390 for the same timestamps, we get the following results: \(\mathrm{CRII}_{2001}/\mathrm{CRII}_{2014}=0.83\) and \(\mathrm{CRII}_{1996}/\mathrm{CRII}_{2009}=0.88\). According to these results, it is clear that as the solar activity increased, the ionization decreased as expected. Additionally, different dynamics were observed between solar cycles 23 and 24, due to the difference in solar activity during these two solar cycles which is indicated in Table 1. Concerning the different FLs, it can be concluded that the higher the FL, the higher the CRII and the radiation exposure of aircrews and frequent flyers, since the provided shielding of the atmosphere was reduced in higher atmospheric altitudes. Comparing the CRII and dH\({}^{*}\)(10)/dt calculations for the four different rigidities/latitudes (0.1 GV, 3.1 GV, 8.5 GV and 14.9 GV), for all three FLs during the entire period 1996-2019, it was noted that the correlation was positive, with the correlation coefficient R\({}^{2}\) = 0.97. From the above analysis, we conclude that both tools gave significant results and can be used in order to study the effect of the ionization and radiation induced by cosmic rays on the environment, space weather, climate change [62,63] and human health [15,22]. Specifically, advances in technology during the last few decades have made air traveling more accessible to everyone. This has led to an increase in the number of flights as well  
Figure 5: Scatter plot of the yearly distribution of the CRII and the ambient dose equivalent rate for the time period 1996–2019 for FL310, FL350, FL390. as an increase in flight altitude, since commercial aircraft are obliged to travel at higher altitudes due to elevated air traffic. The tools mentioned above are of great importance for the assessment of the health effects of the occupational exposure of aviation crews to radiation due to the permanent galactic radiation background. They are also useful for assessing the health effects of potential additional exposure due to sporadic events which cause elevated radiation (such as SEPs and GLEs) [24; 64] and radiation clouds (which are regional radiation enhancements possibly due to photons, GCR and outer-belt relativistic electrons), where the ambient dose equivalent rates are significantly increased [65]. Furthermore, all the extracted results have a great impact on ensuring properly shielded avionic electronic systems, which depends on the levels of ionization and radiation.