SEZIONE DI ROMA III
In the last few decades, there was an enormous progress in our understanding of the Sun. The predictions of the Standard Solar Model (SSM), which is the fundamental theoretical tool to investigate the solar interior, have been tested by solar neutrino experiments and by helioseismology. The deficit of the observed solar neutrino fluxes, reported initially by Homestake and then confirmed by GALLEX, SAGE, GNO, Kamiokande and Super-Kamiokande, generated the so-called "solar neutrino problem" which stimulated a deep investigation of the solar structure. The problem was solved in 2002 when the SNO experiment obtained a direct evidence for flavour oscillations of solar neutrinos and, moreover, confirmed the SSM prediction of the 8B neutrino flux. The future times could be even more interesting. We now reliably know the solar neutrino oscillation probability and we can go back to the original program of solar neutrino studies, i.e. to probe nuclear reactions in the solar core. At the same time, a new "solar abundance problem" has emerged. Recent determinations of the photospheric heavy element abundances indicate that the sun metallicity is lower than previously assumed. Solar models that incorporate these lower abundances are, however, no more able to reproduce the helioseismic results. In this talk, I will review our present knowledge of the Sun and of the properties of solar neutrinos. I will present the results of the latest SSM calculations and I will discuss the status of solar neutrino measurements. I will present the "solar abundance problem" and I will discuss its possible solutions. I will emphasize the importance of detecting neutrinos produced within the CN-NO cycle (CNO and ecCNO neutrinos) and I will discuss the potential of present and future experiments.