By means of dc magnetization and the ac response of Bi2Sr2CaCu2O8 sing
le crystals it is shown that at the dc irreversibility line the vortex
system has no long-range order in the c direction. We find an energy
dissipation peak at 7 Hz for interplane current that takes place at a
temperature well below the irreversibility line. In this sense, the ir
reversibility line marks the temperature where quasi-two dimensional v
ortices are depinned. The experimental data clearly show the different
nature of two dissipation peaks in the susceptibility: one related to
the interplane currents and the other associated with the intraplane
ones.