MSW effects in vacuum oscillations

We point out that for solar neutrino oscillations with the mass-squared difference of Δm2 ∼ 10-10 - 10-9 eV2, traditionally known as ''vacuum oscillation'' range, the solar matter effects are non-negligible, particularly for the low energy pp neutrinos. One consequence of this is that the values of the mixing angle θ and π/2 - θ are not equivalent, leading to the need to consider the entire physical range of the mixing angle 0 (le) θ (le) π/2 when determining the allowed values of the neutrino oscillation parameters