Nonlinear Electrodynamic Birefringence in a Multipole Magnetic Field

A nonlinear electrodynamic change in the phase of an electromagnetic wave after passing through a hexapole magnetic field is determined. It is shown that any pulse of electromagnetic radiation changes its initial polarization due to vacuum nonlinear electrodynamics equations. The front and back of the pulse with the length of c τ are linearly polarized in mutually orthogonal planes; the part enclosed between them, in the general case, becomes an elliptically polarized wave.