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The correlation between Ca and Halpha chromospheric emission, known to bepositive in the solar case, has been found to vary between -1 and 1 for otherstars. Our objective is to understand the factors influencing this correlationin the solar case, and then to extrapolate our interpretation to other stars.We characterize the correlation between both types of emission in the solarcase for different time scales. Then we determine the filling factors due toplages and filaments, and reconstruct the Ca and Halpha emission to testdifferent physical conditions in terms of plage and filament contrasts. We havebeen able to precisely determine the correlation in the solar case as afunction of the cycle phase. We interpret the results as reflecting the balancebetween the emission in plages and the absorption in filaments. We found thatcorrelations close to zero or slightly negative can be obtained whenconsidering the same spatio-temporal distribution of plages and filaments thanon the sun but with greater contrast. However, with that assumption,correlations close to -1 cannot be obtained for example. Stars with a very lowHalpha contrast in plages and filaments well correlated with plages couldproduce a correlation close to -1. This study opens new ways to study stellaractivity, and provides a new diagnosis that will ultimately help to understandthe magnetic configuration of stars other than the sun.

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