It is shown by means of a 2-D MHD simulation that the observed rotation of the unperturbed magnetic field within the magnetopause (MP), which is assumed to be a tangential discontinuity (TD), leads to a significant dependence of the magnetopause Kelvin-Helmholtz (K-H) instability on the orientation of the sheath field. That is, northward sheath BZ is more favorable to the instability than southward BZ and the MP undulates most strongly when the sheath field points due north. When the sheath field does not point due north, a slow rarefaction region is formed in the velocity boundary layer (BL) inside the MP, wherein there are minima of the plasma pressure and temperature accompanied by a maximum of the magnetic pressure. ¿ American Geophysical Union 1995