## 3 Results and Discussion
Without loss of the properties, such a double effect may be tackled at first by iterating the attitude damping factor, thus accounting for a faster decrease in the erosion yield (\(E_{\rm Y}\propto\cos^{2}\alpha\)): the corresponding modified tilt law (blue-dotted curve in Figure 6) describes well the low erosion at an attitude above 45\({}^{\circ}\) but clearly underestimates the AtOx flux effectiveness at intermediate surface orientations. A further parametric correction must then be introduced in order to take into account how the UV irradiation would increase the main AtOx erosion mechanism. Such a synergic effect has been widely observed [12; 17; 18; 19]; nevertheless, due to both the high complexity of the phenomena and the lack of a full,  
Figure 2:0 AtOx/UV erosion of the C/C samples at different attitudes: experimental measurements and physical modeling. reliable database, a systematic analysis of an AtOx-damaging enhancement by concomitant effects (such as surface chemical etching induced by vacuum UV and thermal dilatation) is still under investigation [20]. Preliminarily, by similar considerations made for the AtOx fluence's effectiveness, the UV boosting factor is expected to be dependent on the mutual orientation between the radiating stream and the surface of the material: it is easy to see that, within the adopted test setup (see Figure 1), the AtOx and UV beams are approximately shifted by \(\pi/2\) from each other; thus, the relationship between erosion rate and attitude becomes:  
Figure 2:1  
where the single, dimensionless parameter introduced to estimate the AtOx/UV coupling can be optimized in order to fit the experimental data. A value of \(k_{uv}\sim 0.5\) is retrieved by a standard regression method to minimize the whole discrepancy with the measurements, as shown in Figure 6 (green dashed curve): interestingly, the proposed modeling successfully explains how a relatively low UV exposure could amplify the damage caused by frontal impinging AtOx, whereas a more direct UV irradiation would be almost irrelevant (at least in terms of the material's mass loss) if the AtOx erosion capability is itself suppressed by high-tilted attitude conditions. By recalling Equations (3)-(5), a generalized expression aimed at predicting the full amount of mass eroded during a defined time/orbit exposure to the AtOx/UV environment is then given as:  
Figure 2:2  
which, beyond the constitutive structure's parameters (density, exposed area, and nominal AtOx erosion rate), requires knowing the AtOx flux and the UV-enhancing factor for each time step \(i\), as well as the time-variation attitude with respect to both the orbital motion and the UV source (\(\hat{\mathbf{v}}_{uv}\)) along each mission phase. The analysis presented here should be of aid for gaining clues to the well-known discrepancies between AtOx in-flight experiments and ground simulation results, which are frequently up to one order of magnitude or more and in both directions [13; 17; 18; 19; 20; 21; 22]. For example, in [13], a value of an AtOx reactivity coefficient of \(0.63\times 10^{-24}\) cm\({}^{3}\)/n.at is reported as delivered in the spacecraft flow for STS-8 for basal-oriented graphite material, while a much greater result of \(9.2\times 10^{-24}\) cm\({}^{3}\)/n.at. is given by the ground simulation experiment; in contrast, the same benchmark review reports values in the range of \(2.0\)-\(4.7\times 10^{-25}\) cm\({}^{3}\)/n.at. as found in the LDEF spacecraft flow for FEP Teflon specimens, while a reduced coefficient of \(0.7\)-\(0.8\times 10^{-25}\) cm\({}^{3}\)/n.at is reported by the ground simulation experiment.