Estimation of the spatial coherence outer scale for daytime observations

The theoretical expression of the transverse angular covariance of angle-of-arrival fluctuations of the incoming wave-fronts integrated all over the telescope pupil has been established in the case of one-layer model and using the Von Kàrmàn theory. This model depends on three atmospheric parameters: the spatial coherence outer scale ${\cal L}_0$, the Fried parameter $r_0$ and the size of the isoplanatic patch $\theta_0$. We use this model to analyze the fluctuations observed on the solar limb, which finally consists on the estimation of the three parameters ${\cal L}_0$, $r_0$ and $\theta_0$. The value of the spatial coherence outer scale ${\cal L}_0$ for day-time observations is not known. Its estimation is particularly important in the modelisation of atmospheric effects for solar observations with high angular resolution (solar diameter measurements at astrolabe for example). Two methods have been developed in other to estimate these parameters. They have been investigated and applied to solar images recorded at Calern Observatory Astrolabe (France). It is shown that the values obtained for ${\cal L}_0$ are smaller than those found for night observations. The obtained results are presented and discussed.