  The general formalism for the description of neutrino oscillations in
arbitrary rapidly varying external fields is elaborated. We obtain the new
effective Hamiltonian which determines the evolution of the averaged neutrino
wave function. The general technique is applied to the neutrino oscillations in
rapidly varying magnetic fields. We evaluate the transition probabilities of
the neutrino spin-flavor oscillations in magnetic fields of the Sun and compare
them with the numerical solutions of the Schroedinger equation with the exact
Hamiltonian.
