  If massive neutrinos possess magnetic moments, they can undergo spin flip in
a magnetic field. The magnetic fields needed for a meaningful measurement of
neutrino moments could be very high and may occur in astronomical objects such
as some supernovae or active galactic nuclei: they are typically chaotic ones.
We develop the general theory of the passage of neutrinos through such fields.
We also develop a simple model which becomes solvable in the high energy limit.
Both helicities occur with equal probability, independently of the initial
distribution. Observational consequences are discussed.
