  The Josephson vortices dynamic in superconducting $YBa_2Cu_3O_x$ ceramics was
studied in frequency range 0.01-0.5 Hz by ac susceptibility measurements in non
complete magnetic flux penetration regime. We have found that with the increase
of magnetic field frequency starting from 0.01 Hz the ac magnetic response
shows complex frequency dependence: in one half period of a sinusoidal field
appear two asymmetrical peaks of real $\Delta\chi\prime$ and imaginary
$\chi\prime\prime$ parts of the ac susceptibility. The analysis of experimental
results has led to an idea that probably the volume (diameter) of Josephson
vortices increases during its movement. The experimental results are discussed
in terms of Josephson vortex - vortex interactions.
