  We report results of experimental studies of ac susceptibility of Nb single
crystal at low frequencies in swept magnetic fields applied parallel to the
surface. Analysis of the experimental data shows that the swept magnetic field
significantly changes the vortex conductivity in the bulk. It becomes
dissipative with unexpected large frequency dispersion. At the surface, one
observes a layer with enhanced conductivity in comparison to the bulk. This
layer provides a considerably large contribution to the shielding and
absorption of an ac field even in dc fields below $H_{c2}$. We have also
demonstrated that the swept magnetic field apparently affects an ac response of
the surface superconducting states.
