The thickness of the magnetopause boundary layer can be explained by anomalous cross-field diffusion of magnetosheath protons resonating with either electrostatic or electromagnetic waves (Tsurutani and Thorne, 1982). Whereas the first mechanism was favored by these authors on the basis or ISEE measurements, GEOS data, which are obtained when the magnetosphere is contracted, seem to show that the second one may be more important, at least during disturbed conditions. The special relationship which exists between the proton resonant energy and the frequency of right-handed electromagnetic waves leads to a preferential diffusion of protons of a few keV. The ability of electromagnetic waves to evacuate the energy dissipated during the generation of turbulence is also discussed.