Observation of nearly undisturbed penetration of the Porcupine dense (nb≫nion) and fast (vb=1.7 ⋅ 104 ms-1) heavy (xenon, mi=131 mp) ion beam into the magnetized ionospheric plasma after termination of the short (&tgr;m<1 ms) adiabatic phase requires very fast magnetic field diffusion into the beam. Collisional diffusion and instability of a sharp boundary between the magnetic field and plasma yield diffusion times too long to account for the observations. The diffusion is therefore attributed to a transverse electron drift current--driven electrostatic instability excited by the diamagnetic current flowing in the boundary layer between the injected beam and the ambient field. In the low-drift regime applicable to the final stage of diffusion in the outer part of the layer, electron acoustic waves are excited in the initial strong current regime of the inner part of the layer the modified two-stream instability becomes unstable. The anomalous collision frequencies produced in both cases by entirely different saturation mechanisms turn out to be of the order of the local lower-hybrid (LH) frequency in the dense xenon plasma and are high enough to yield the required fast magnetic field diffusion rate. Energetically, only a very small fraction of beam energy is dissipated in the diffusion process so that no remarkable deceleration of the ion beam is observed, thus giving a satisfactory explanation of the negigible beam energy losses.