Solar Neutrinos and Grand Unification

We explore possible resolutions of the solar neutrino problem in the context of Grand Unified Theories. It is known that GUTs which break directly to the Standard Model can give rise to a pattern of neutrino masses with $(\Delta m^2)_{13} \sim (10^{-6}-10^{-5}) eV^2$ which is in the right range for the MSW solution via $(\nu_e-\nu_\tau)$ conversion. We show that the $(\nu_e-\nu_\mu)$ mass splitting in such schemes can naturally be in the range $(\Delta m^2)_{12} \sim (10^{-11}-10^{-9}) eV^2$. Moreover, the $(\nu_e-\nu_\mu)$-mixing can be sufficiently large ($\sin^2 2\theta_{e\mu} > 0.2$) so that an interplay of the matter resonance conversion and vacuum oscillations of solar neutrinos occurs. New ranges of neutrino parameters which provide fits to the solar neutrino data are found. This hybrid (MSW+vacuum oscillations) scenario leads to peculiar distortions of energy spectrum of the boron neutrinos which should be observable at the SuperKamiokande and SNO experiments.