  The study of solar neutrinos may provide important insights into the physics
of the central region of the Sun. Four solar neutrino experiments have
confirmed the solar neutrino problem but do not clearly indicate whether solar
physics, nuclear physics, or neutrino physics have to be improved to solve it.
Nonlinear relations among the different neutrino fluxes are imposed by two
coupled systems of differential equations governing the internal structure and
time evolution of the Sun. We assume that the results of the four neutrino
experiments are correct and are concerned not with the discrepancy between the
average rate and the predicted rate, but with a possible time dependence of the
argon production rate as revealed in the Homestake experiment over a time
period of 20 years. Based on the subtlety of the solar neutrino problem we
review here quantitatively the physical laws employed for understanding the
internal solar structure and conjecture that the interlink between specific
nuclear reactions of the PPIII-branch of the proton-proton chain may allow the
high-energy solar neutrino flux to vary over time.
