PITCH ANGLE SCATTERING IN THE OUTER HELIOSHEATH AND FORMATION OF THE INTERSTELLAR BOUNDARY EXPLORER RIBBON

The dominant and unexpected feature in the first Interstellar Boundary EXplorer (IBEX) maps is a ribbon of the enhanced energetic neutral atom (ENA) emissions. Presenting the first results from IBEX, McComas et al. identified six possible mechanisms of ribbon formation. One of the mechanisms, the so-called secondary ENA mechanism, was already quantitatively elaborated by Heerikhuisen et al., and they successfully reproduced the main features of the ribbon. We further study the 'secondary ENA' mechanism by quantifying a previously omitted stage of the proton evolution between two consecutive acts of the charge-exchange in the outer heliosheath (OHS). The main findings can be summarized as follows. (1) The neutrals supplied by the supersonic near-equatorial solar wind dominate the near-equatorial source of the keV ENAs in the OHS compared to the inner heliosheath contribution. (2) The ribbon of the observed width can be produced even if only the large-scale (∼102-104 AU) interstellar turbulence operates but the resulting pitch angle distribution functions (PADFs) are unstable with respect to the ion cyclotron wave generation around the locus where the line of sight from IBEX to the ribbon is perpendicular to the interstellar magnetic field beyond the heliopause. (3) A combination of the large-scale interstellar turbulence and a small-scale (∼10-5to10-4 AU) turbulence generated by an unstable PADF of the energetic protons is able to make PADF marginally stable. In this case, the ribbon is still narrow because only a small part of the proton phase space distribution function can resonate with a locally generated ion cyclotron turbulence. (4) A concurrent operation of the large-scale interstellar turbulence and the locally generated small-scale turbulence might be responsible for the localized emission structures observed in the IBEX ribbon.