  We discuss various aspects of the solar neutrino spectrum distortion and time
variations of fluxes. (i) Oscillations of neutrinos which cross the mantle and
the core of the Earth can be parametrically enhanced. The parametric effect
gives correct physical interpretation of the calculated day-night asymmetry.
(ii) Solution of the $\nu_{\odot}$-problem in schemes with three and more
neutrinos which accommodate explanations of other neutrino anomalies, in
particular, the atmospheric neutrino anomaly, can lead to complicated
distortion of the boron neutrino spectrum. (iii) The study of correlations
between time (seasonal or day-night) variations and spectrum distortion will
help to identify the solution of the $\nu_{\odot}$-problem.
