Solar neutrinos: beyond standard solar models

After a short survey of the physics of solar neutrinos, giving an overview of hydrogen burning reactions, predictions of standard solar models and results of solar neutrino experiments, we discuss the solar-model-independent indications in favour of non-standard neutrino properties. The experimental results appear to be in contradiction with each other, even after disregarding some experiments: unless electron neutrinos disappear in their trip from the sun to the earth, the fluxes of intermediate energy neutrinos (those from 7Be electron capture and from the CNO cycle) turn out to be unphysically negative, or extremely reduced with respect to standard solar model predictions. Next we review extensively the non-standard solar models built as attempts to solve the solar neutrino puzzle. The dependence of the central solar temperature on chemical composition, opacity, age and on the values of the astrophysical S-factors for hydrogen-burning reactions is carefully investigated. Also, possi...