Resonant spin-flavor precession of solar and supernova neutrinos

The combined effect of matter and magnetic fields on neutrino spin and flavor precession is examined. We find a potential new kind of resonant or (for Dirac neutrinos) or νₑ{→}{ν}_μ or {ν}_τ (for Majorana neutrinos). Such a resonance could help account for the lower than expected solar-neutrino νₑ flux and/or indications of an anticorrelation between fluctuations in the νₑ flux and sunspot activity. Consequences of spin-flavor precession for supernova neutrinos are also briefly discussed.