The Mössbauer spectra of a permalloy film placed in an rf magnetic field are measured. The film is so thin that transverse magnetostriction oscillations are not excited in it. This permits the observation of satellites that are due solely to periodic variations of the magnetic field at the nucleus. The data are in good agreement with calculations performed in the model of step-wise field reversals. On application of an additional static magnetic field the splitting of the satellites is observed, as predicted by the model of asymmetric step-wise field reversals.