Introduction

Solar neutrino experiments provide unique information on neutrino propagation in vacuum and in matter. On the way from the central parts of the sun to the terrestrial detectors, the neutrinos can decay, oscillate, undergo resonant flavor conversion, or have their spins precess or flip resonantly. These processes may be manifested in different ways, including an energy-dependent suppression of the electron neutrino flux, the appearance of muon or tau (anti)neutrinos or even electron antineutrinos from the sun, and time variations of the fluxes.