Coherent conversion of neutrino flavor by collisions with relic neutrino gas

In this paper, we extend considerations developed for application to coherence in spontaneous radiation processes involving a gas, to interactions of high energy neutrinos with a relic neutrino gas leading to conversion of flavor. Under certain conditions we show that this flavor conversion can be strongly enhanced by coherence effects. These considerations are applied to neutrinos from the Sun and from Supernovae, and estimates for the enhancement effects are given.