  Vortex matter phases and phase transitions are investigated by means of
Josephson plasma resonance in under-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$
single crystals in a microwave frequency range between 19 and 70 GHz.
Accompanied by the vortex lattice melting transition, a jump of the interlayer
phase coherence extracted from the field dependence of the plasma frequency was
observed. In the solid phase, the interlayer coherence little depends on field
at a temperature region well below $T_c$ while it gradually decreases as field
increases toward the melting line up to just below $T_c$. As a result, the
magnitude of the jump decreases with increasing temperature and is gradually
lost in the vicinity of $T_c$. This indicates that the vortex lines formed in
the vortex solid phase are thermally meandering and the phase transition
becomes weak especially just below $T_c$.
