Solar-neutrino problem: Some old solutions reexamined

Recent experimental data confirm the solar-neutrino problem and imply new neutrino physics. We review some of the less discussed proposals to solve this problem: (1) maximal vacuum mixing of three neutrino flavors, (2) vacuum oscillation of two neutrino flavors, and (3) neutrino decay. Each of these three solutions can fit the ${}_{}{}^{37}{}_{}{}^{}\mathrm{Cl}$ and Kamiokande-II solar-neutrino-flux measurements. Their implications for the continuing ${}_{}{}^{71}{}_{}{}^{}\mathrm{Ga}$ experiments and for other, future experiments are discussed.