We investigate the nonlinear influence of the cross-tail currents carried by beamlets (substructures of PSBL ion beams) on the topology of the magnetic field, and, correspondingly, on the dispersion properties of these substructures self-consistently generated in this field. We found that some of the peculiarities of beamlet shapes found recently in CLUSTER data could be explained by taking into account the nonlinearity of the system. This model explains the steepening of local beamlets dispersion in comparison with the global dispersion of the enveloping VDIS structure. At the same time we found that velocity filter effects operating during beamlets propagation toward the Earth prevent the sign's reversal of this local dispersion.