Diffractive-like (or parametric-resonance-like?) enhancement of the Earth (day-night) effect for solar neutrinos crossing the Earth core

It is shown that the strong enhancement of the Earth (day-night) effect for solar neutrinos crossing the Earth core in the case of the small mixing angle MSW nu(e) --> nu(mu(tau)) transition solution of the solar neutrino problem is due to a new resonance effect in the solar neutrino transitions in the Earth and not just to the MSW effect in the core. The effect is in many respects similar to the electron paramagnetic resonance. The conditions for existence of this new resonance effect are discussed. They include specific constraints on the neutrino oscillation lengths in the Earth mantle and in the Earth core, thus the resonance is a "neutrino oscillation length resonance". The effect exhibits strong dependence on the neutrino energy. Analytic expression for the probability accounting for the solar neutrino transitions in the Earth, which provides a high precision description of the transitions, including the new resonance effect, is derived. The implications of our results for the searches of the day-night asymmetry in the solar neutrino experiments are also briefly discussed. The new resonance effect is operative also in the nu(mu) --> nu(e) (nu(e) --> nu(mu)) transitions of atmospheric neutrinos crossing the Earth core. (C) 1998 Elsevier Science B.V. All rights reserved.