  It is shown that under the action of rotating magnetic field an immobile
vortex, contrary to a general belief, can nucleate a vortex-antivortex pair and
switch its polarity. Two different kinds of OOMMF micromagnetic modeling are
used: (i) the original vortex is pinned by the highly--anisotropic easy-axes
impurity at the disk center, (ii) the vortex is pinned by artificially fixing
the magnetization inside the vortex core in the planar vortex distribution. In
both types of simulations a dip creation with a consequent vortex-antivortex
pair nucleation is observed. Polarity switching occurs in the former case only.
Our analytical approach is based on the transformation to the rotating frame of
reference, both in the real space and in the magnetization space, and on the
observation that the physical reason for the dip creation is softening of the
dipole magnon mode due to magnetic field rotation.
