## 3 Results  
In this work, a study of cosmic-ray-induced ionization, computed via the CRAC:CRII model [12; 13], along with the estimated ambient dose equivalent rate computed via the validated software DYNASTIMA/DYASTIMA-R [39; 40], was performed globally during the last two solar cycles (23 and 24) and focused on specific altitudes that corresponded to the most common commercial flight levels: FL310 (9.45 km a.s.l. ), FL350 (10.67 km a.s.l.) and FL390 (11.89 km a.s.l.). More specifically, the CRII at FL390 during the solar minima and solar maxima of solar cycles 23 and 24 is presented in Figure 1, globally, via ionization maps. Figure 1a depicts the CRII map that corresponds to the minimum of solar cycle 23 (in the year 1996), Figure 1b depicts the CRII map that corresponds to the maximum of solar cycle 23 (in the year 2001), Figure 1c depicts the CRII map that corresponds to the minimum of solar cycle 24 (in the year 2009) and Figure 1d depicts the CRII map that corresponds to the maximum of solar cycle 24 (in the year 2014). Comparing these four maps, it is clear that the ionization rate during the solar minima was greater than the ionization rate during the solar maxima of both cycles. This is due to the fact that the CRII followed the behavior of the cosmic ray intensity and was positively correlated with them, while it negatively correlated with the solar activity. In other words, the greater the solar activity, the lower the intensity of the CRII is [57; 58; 59]. Moreover, when comparing the solar minima and maxima of solar cycles 23 and 24, it is obvious that the CRII had greater values during solar cycle 24 than that of solar cycle 23, which was well expected, since solar cycle 24 is characterized as a relatively quiet solar cycle, unlike solar cycle 23, where the solar activity was greater. The minimum and maximum values for CRII and dH*(10)/dt obtained during this work for these specific time periods are presented in Table 1. Regarding the geographic coordinates, it was observed that, globally, the maximum ionization rate was found in polar regions while, at lower latitudes, the ionization rate reached minimum. This was due to the magnetic field of the Earth and the geomagnetic cut-off rigidity (Rc) that corresponded to each location, from 0 GV in polar regions to up to 17 GV in equatorial regions. The lower the geomagnetic cut-off rigidity (Rc), the more cosmic rays penetrated the magnetosphere and the atmosphere of the Earth; the CRs then ionized the atmosphere and created various effects [51]. Both the CRII and estimated ambient dose equivalent rate maps were generated based on the rigidity map of [53; 54; 55; 56]. Figure 1: _Cont._  
Figure 1: Maps of the CRII rate (ion pairs/(g*s)) at FL390: (**a**) during the minimum of solar cycle 23; (**b**) during the maximum of solar cycle 23; (**c**) during the minimum of solar cycle 24; (**d**) during the maximum of solar cycle 24. With regard to the radiation exposure, the ambient dose equivalent rate at FL390 during the solar minima and solar maxima of solar cycles 23 and 24 is presented in Figure 2. A behavior similar to that of CRII was noticed. Greater values of the dH*(10)/dt were observed in the polar regions (Rc = 0-2 GV) and lower values near the equator (Rc = 15-17 GV), for both solar minima and maxima conditions. This was due to the dependence of the radiation levels at the atmospheric layers on the cosmic ray intensity [40; 60]. As expected, the radiation exposure was greater during the solar minima compared to the solar maxima, due to the negative correlation between the solar activity and the intensity of the incoming cosmic ray particles. Greater values of dH*(10)/dt were also observed during the extended solar minimum in 2009 for both polar and equatorial regions, as compared to those observed during 1996.