  We use a hydrodynamics theory to discuss the response of a viscous flux-line
liquid to an ac perturbation applied at the surface of the sample. The theory
incorporates viscoelastic effects and describes the crossover between
liquid-like and solid-like response of the vortex array as the frequency of the
perturbation increases. A large viscosity from flux-line interactions and
entanglement leads to viscous screening of surface fields. As a result, two
frequency-dependent length scales are needed to describe the penetration of an
ac field. For large viscosities the imaginary part of the ac permeability can
exihibit, in addition to the well-know peak associated with flux diffusion
across the sample, a new low-frequency peak corresponding to the transition
from solid-like to liquid-like behavior.
