Illustration of the ribbon formation as a result of a neutral density enhancement. Left panel: the blue lines indicated the heliopause, the black lines give the orientation of the undisturbed interstellar magnetic field (ISM) along which a neutral density enhancement (H-wave, see text) with a characteristic scale of a few tens of AU is propagating. Since the actual extent of such disturbance, which is schematically illustrated here by the green cylindrical area, in directions perpendicular to the magnetic field is not important in the present context, the cylinder indicates that part of the disturbance that is of interest for the ribbon formation. Right panel (reproduced by permission of the AAAS): the flux of energetic neutral atoms projected onto the heliopause around which the interstellar magnetic field lines are draping (adopted from McComas et al. 2009b).