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 $Deltachiprime$ and imaginary $chiprimeprime$ 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.
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