  We report on the degree of order of the vortex solid in YBa2Cu3O7 single
crystals observed in ac susceptibility measurements. We show that when vortices
are "shaken" by a temporarily symmetric ac field they are driven into an
easy-to-move, ordered structure but, on the contrary, when the ac field is
temporarily asymmetric, they are driven into a more pinned disordered state.
This is characteristic of tearing of the vortex lattice and shows that ordering
due to symmetric ac fields is essentially different from an equilibration
process or a dynamical crystallization that is expected to occur at high
driving currents.
