Using the records of the fluxes of solar neutrinos from the Homestake, GALLEX (GNO), and Super-Kamiokande experiments, their statistical analyses were performed to search for whether there existed a time variation of these fluxes. The results of the
analysis for the three experiments indicate that these fluxes are varying quasi-biennially. This means that both efficiencies of the initial p-p and the pp-III reactions of the proton-proton chain are varying quasi-biennially together with a period of about 26 months. Since this time variation prospectively generated by these two reactions strongly suggests that the efficiency of the proton-proton chain as the main energy source of the Sun has a tendency to vary quasi-biennially due to some chaotic or non-linear process taking place inside the gravitationally stabilized solar fusion reactor. It should be, however, remarked that, at the present moment, we have no theoretical reasoning to resolve this mysterious result generally referred to as the quasi-biennial periodicity in the time variation of the fluxes of solar neutrinos. There is an urgent need to search for the reason why such a quasi-biennial periodicity is caused through some physical process as related to nuclear fusion deep inside the Sun.
