  The correlation between Ca and Halpha chromospheric emission, known to be
positive in the solar case, has been found to vary between -1 and 1 for other
stars. Our objective is to understand the factors influencing this correlation
in the solar case, and then to extrapolate our interpretation to other stars.
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 Halpha emission to test
different physical conditions in terms of plage and filament contrasts. 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
Halpha contrast in plages and filaments well correlated with plages could
produce a correlation close to -1. 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.
