  We report complex impedance measurements in an untwinned YBaCuO crystal. Our
broad frequency range covers both the quasi static response and the resistive
response of the vortex lattice. It allow us to characterize the irreversibility
line without the need of any frequency dependent pinning parameters. We confirm
the validity of the two modes model of vortex dynamic, and extract both the
surface critical current and the flux flow resistivity around the first order
transition $T_{m}$. This latter is identified by the abrupt loss of pinning and
by an unexpected step of $\rho_{ff}(T)$ at $T_{m}$.
