Study of photospheric line depth variations along the solar cycle

We study the behaviour of three photospheric lines (Fe I 537.9, C I 538.0 and Ti II 538.1 nm), monitored on the Sun since 1978, either as full-disk or as center-disk measurements. The aim is to detect photospheric variations with the cycle. We reconstruct the cyclic variations of full-disk line depths as due to the active region modulation, through a spectral synthesis with FAL semiempirical models (Fontenla et al.). We show that ARs alone cannot account all the observational results. The difference between observed behaviour of these three lines at full-disk and the AR contributionm as predicted by the models, correlates with the measured center-disk line variations, and a common periodicity of similar to 2.8 yr is present.