[en] The results of researchers of dynamics of particles in a constant magnetic field and in a field of a wave, which amplitude varies with the given frequency are stated. It is shown, that if the frequency of external modulation will be close to the double of buns-frequency of particles, which are seized by a field of wave, then the parametric instability arise. It is shown, that if the frequency of modulation considerably surpasses buns-frequency of particles, unstable saddle point can become stable. The possible consequences of these features of dynamics of particles are discussed.