Scatter-free pickup ions beyond the heliopause as a model for the interstellar boundary explorer ribbon

We present a new kinetic-gasdynamic model of the solar wind interaction with the local interstellar medium. The model incorporates several processes suggested earlier for the origin of the ribbon - the most prominent feature seen in the all-sky maps of heliospheric energetic neutral atoms (ENAs) discovered by the Interstellar Boundary Explorer (IBEX). The ribbon is a region of enhanced fluxes of ENAs crossing almost the entire sky. Soon after the ribbon's discovery, it was realized that the enhancement of the fluxes could be in the directions where the radial component of the interstellar magnetic field around the heliopause is close to zero. Our model includes secondary charge exchange of the interstellar H atoms with the interstellar pickup protons outside the heliopause. Previously, in the frame of a kinetic-gasdynamic model where pickup protons are treated as a separate kinetic component, it was shown that the interstellar pickup protons outside the heliopause may be a significant source of ENAs at energies above 1 keV. The key difference between the current work and the previous models is in the assumption of no pitch-angle scattering for newly created pickup protons outside the heliopause. We demonstrate that in the limit of no pitch-angle scattering ribbon of enhanced ENA fluxes appears in the model, and this may qualitatively explain the ribbon discovered by IBEX.