  We need to establish a correspondence between the magnetic structures
generated by models and usual stellar activity indexes to simulate radial
velocity time series for stars less active than the Sun. This is necessary to
compare the outputs of such models with observed radial velocity jitters and is
critical to better understand the impact of stellar activity on exoplanet
detectability. We propose a coherent picture to describe the relationship
between magnetic activity, including the quiet Sun regions, and the
chromospheric emission using the Sun as a test-bench and a reference. We
analyzed a time series of MDI magnetograms jointly with chromospheric emission
time series obtained at Sacramento Peak and Kitt Peak observatories. This has
allowed us to study the variability in the quiet Sun over the solar cycle, and
then, based on available relations between magnetic fields in active structures
and chromospheric emission, to propose an empirical reconstruction of the solar
chromospheric emission based on all contributions. We show that the magnetic
flux covering the solar surface, including in the quieted regions, varies in
phase with the solar cycle, suggesting a long-term relationship between the
global dynamo and the contribution of all components of solar activity. We have
been able to propose a reconstruction of the solar S-index, including a
relationship between the weak field component and its chomospheric emission,
which is in good agreement with the literature. This allows us to explain that
stars with a low average chromospheric emission level exhibit a low
variability. We conclude that weak flux regions significantly contribute to the
chromospheric emission; these regions should be critical in explaining the
lower variability associated with the lower average activity level in other
stars as compared to the Sun and estimated from their chromospheric emission.
