Solar neutrinos and 1-3 leptonic mixing

Effects of the 1-3 leptonic mixing on the solar neutrino observables are studied and the signatures of non-zero $\theta_{13}$ are identified. For this we have re-derived the formula for $3\nu$-survival probability including all relevant corrections and constructed contours of constant values of observables in the $\sin^2\theta_{12} - \sin^2\theta_{13}$ plane. Analysis of the solar neutrino data gives $\sin^2\theta_{13} = 0.02^{+0.05}_{-0.02}$ ($1\sigma$) for $\Delta m^2 = 8\cdot 10^{-5}$ eV$^2$. The combination of the ratio CC/NC at SNO and gallium production rate selects $\sin^2\theta_{13} = 0.034 \pm 0.026$ ($1\sigma$). The global fit of all oscillation data leads to the best value $\sin^2\theta_{13} = 0.004$. The sensitivity of future solar neutrino studies to $\sin^2\theta_{13}$ can be improved down to 0.01 - 0.02 ($1\sigma$) by precise measurements of the pp-neutrino flux and the CC/NC ratio as well as spectrum distortion at high ($E > 5$ MeV) energies. We show that combination of experimental results sensitive to the low and high energy parts of the solar neutrino spectrum resolves the degeneracy of angles $\theta_{13}$ and $\theta_{12}$. Comparison of $\sin^2\theta_{13}$ as well as $\sin^2 \theta_{12}$ measured in the solar neutrinos and in the reactor/accelerator experiments may reveal new effects which can not be seen otherwise.