Testing the H-wave Hypothesis for the Origin of the IBEX Ribbon

Abstract

The Interstellar Boundary Explorer (IBEX) spacecraft maps the neutral atom fluxes across the whole sky. Thereby it is indirectly mapping the structure of the outer heliosphere and the (very) local interstellar medium. A particularly interesting feature in the IBEX-Hi all-sky maps of the differential flux of Energetic Neutral Atoms (ENAs) in the 0.7 to 4.3 keV range is the so-called ribbon, i.e. a factor of two to three enhancement in a twenty to thirty degree wide band across the sky. Amongst other hypotheses it has been argued that this ribbon may be related to a neutral density enhancement, a so-called H-wave, in the local interstellar medium. By employing an analytical model of the large-scale structure of the heliosphere it is demonstrated that the H-wave scenario for the ribbon formation leads to results that are fully consistent with the observed location of the ribbon in the full-sky maps as well as in space at all energies detected with IBEX-Hi. As a further extension to previous work and as a necessary prerequisite for the computation of differential ENA fluxes, the evolution of the proton velocity distribution function in the inner heliosheath is semi-analytically computed in terms of a K-functions with locally determined K-values.