For small mixing angles θ the amplification of ν oscillations in matter has the resonance form (resonance in neutrino energy or matter density). In the Sun resonance effect results in nontrivial changing (suppression) of ν-flux for a wide range of neutrino parameters Δm 2=(3·10−4÷10−8) (eV)2, sin22θ>10−4.

Matter can enhance neutrino oscillations (increase the mixing parameter sin2 2θ m ). For small mixing angles in vacuum the enhancement displays a resonance behavior in the neutrino energies or the density of the medium. This resonance effect is important for solar neutrinos in a wide range of oscillation parameters Δm 2 = 10-4-10-8 eV2 and sin2 2θ > 10-4. It leads to a strong suppression of the neutrino flux even for small sin2 2θ.