On the correlation between Ca and Hα solar emission and consequences for stellar activity observations

Context. The correlation between Ca and H alpha chromospheric emission, known to be positive in the solar case, has been found to vary between -1 and 1 for other stars. Aims. Our objective is to understand the factors influencing this correlation in the solar case, and then to extrapolate our interpretation to other stars. Methods. We characterize the correlation between both types of emission in the solar case for different time scales. Then we determine the filling factors due to plages and filaments, and reconstruct the Ca and H alpha emission to test different physical conditions in terms of plage and filament contrasts. Results. We have been able to precisely determine the correlation in the solar case as a function of the cycle phase. We interpret the results as reflecting the balance between the emission in plages and the absorption in filaments. We found that correlations close to zero or slightly negative can be obtained when considering the same spatio-temporal distribution of plages and filaments than on the sun but with greater contrast. However, with that assumption, correlations close to -1 cannot be obtained for example. Stars with a very low H alpha contrast in plages and filaments well correlated with plages could produce a correlation close to -1. Conclusions. This study opens new ways to study stellar activity, and provides a new diagnosis that will ultimately help to understand the magnetic configuration of stars other than the sun.