We propose that the intense neutrino flux emitted by the core of a supernova generates parametric decay instabilities (neutrinos generate plasma waves and lower energy neutrinos) in the surrounding stellar envelope. The process is analogous to the stimulated Raman instability associated with the interaction of intense light with plasma. We develop a theory for the instability based on the index of refraction of neutrinos. Our results indicate that the instability is important and should play a role in supernova dynamics.
