MSW Effects in Vacuum Oscillations

We point out that for solar neutrino oscillations with the mass-squared difference of Delta m^2 ~ 10^-10 - 10^-9 eV^2, 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 theta and pi/2-theta are not equivalent, leading to the need to consider the entire physical range of the mixing angle 0<=theta<=pi/2 when determining the allowed values of the neutrino oscillation parameters.