  The effect of a magnetic field component parallel to the superconducting
layers on longitudinal Josephson plasma oscillations in the layered high
temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ is shown to
depend on the thermodynamic state of the underlying vortex lattice. Whereas the
parallel magnetic field component depresses the Josephson Plasma Resonance
(JPR) frequency in the vortex solid phase, it may enhance it in the vortex
liquid. There is a close correlation between the behavior of microwave
absorption near the JPR frequency and the effectiveness of pancake vortex
pinning, with the enhancement of the plasma resonance frequency occurring in
the absence of pinning, at high temperature close to the vortex melting line.
An interpretation is proposed in terms of the attraction between pancake
vortices and Josephson vortices, apparently also present in the vortex liquid
state.
