The time variation of the elastic scattering rate of solar neutrinos with electrons in Super-Kamiokande-I was fit to the variations expected from active two-neutrino oscillations. The best fit in the large mixing angle solution has a mixing angle of ${\tan }^2\theta =0.55\mathrm{}$ and a mass squared difference of $\Delta m^2=6.3\mathrm{}\times {10}^{-5}{\mathrm{eV}}^2$ between the two neutrino mass eigenstates. The fitted day-night asymmetry of $-1.8\pm 1.6\left(\mathrm{stat}{)}_{-1.2}^{+1.3\mathrm{}}\right(\mathrm{syst})\%$ has improved statistical precision over previous measurements and agrees well with the expected asymmetry of $-2.1\%.$

• Received 4 September 2003

DOI:https://doi.org/10.1103/PhysRevD.69.011104