We have studied the isothermal, magnetic field(H‖c)dependent rf powerP(H)dissipation(Hrf‖a)in the superconducting state ofBi2Sr2CaCu2O8single crystals prior to and after irradiation with 250 MeV107Ag17+ions. In the pristine state,P(H)shows an initial decrease with increase in field, reaches a minimum atHM(T)and increases monotonically forH>HM(T).This behavior arises when the electromagnetic coupling between the pancake vortices in adjacent CuO layers becomes dominant on increasing the field and minimizes the distortions of the flux lines by confining the 2D vortices. In the post irradiated state, such an initial decrease and the minimum inP(H)is not observed but only a much reduced rf dissipation that monotonically increases with field fromH=0onwards is seen. We attribute this difference to the strong enhancement of the tilt modulusC44of the flux lines on irradiation when the pancake vortices in adjacent CuO bilayers are pinned along the track forming a well-stacked flux line in the field direction(‖c).We have also observed that the rf dissipation disappears at a certain temperatureTsf,at which the normal core of the flux line becomes commensurate with the columnar track diameter. ©1998 American Institute of Physics.