 X-ray reflectivity from a mirror surface at small incidence angles may also be used to generate long-period XSWs.The spatial modulation of the XSW field, described by the dynamical theory of X-ray diffraction, undergoes a pronounced change when the sample is scanned through the Bragg condition.Due to a relative phase variation between the incoming and reflected beams, the nodal planes of the XSW field shift by half the XSW period.