The Super - Kamiokande Day - Night Effect Data and the MSW Solutions of the Solar Neutrino Problem

The current Super-Kamiokande data on the D-N asymmetry between the the day event rate and the Night and Core event rates, produced by solar neu-trinos which respectively cross the Earth along any trajectory and cross the Earth core before reaching the detector, imply rather stringent constraints on the MSW small mixing angle (SMA) ν e → ν µ(τ) solution of the solar neutrino problem. A simplified analysis shows, in particular, that a substantial subre-gion of the SMA solution region is disfavored by these data. The Core D-N asymmetry data alone allow to rule out at 99.7% C.L. a part of this subregion. The constraints on the MSW large mixing angle and LOW ν e → ν µ(τ) solutions as well as on the MSW ν e → ν s solution, following from the data on the Night and Core D-N asymmetries are also discussed.