Matter effects in active-sterile solar neutrino oscillations

We study the matter effects for solar neutrino oscillations in a general scheme, without any constraint on the number of sterile neutrinos and the mixing matrix elements, only assuming a realistic hierarchy of neutrino squared-mass differences in which the smallest squared-mass difference is effective in solar neutrino oscillations. The validity of the analytic results is illustrated with a numerical solution of the evolution equation in the simplest case of four-neutrino mixing with the realistic matter density profile inside the Sun.