  We consider the magnetic field dependence of the plasma resonance frequency
in pristine and in irradiated Bi$_2$Sr$_2$CaCu$_2$O$_8$ crystals near $T_c$. At
low magnetic fields we relate linear in field corrections to the plasma
frequency to the average distance between the pancake vortices in the
neighboring layers (wandering length). We calculate the wandering length in the
case of thermal wiggling of vortex lines, taking into account both Josephson
and magnetic interlayer coupling of pancakes. Analyzing experimental data, we
found that (i) the wandering length becomes comparable with the London
penetration depth near T$_{c}$ and (ii) at small melting fields ($< 20$ G) the
wandering length does not change much at the melting transition. This shows
existence of the line liquid phase in this field range. We also found that
pinning by columnar defects affects weakly the field dependence of the plasma
resonance frequency near $T_c$.
