Physics derived from IBEX ENA fluxes and direct interstellar neutral measurements

Abstract

The Interstellar Boundary Explorer (IBEX) mission has observed the global interaction of our heliosphere with the local interstellar medium (LISM) for over three years. Initially, IBEX generated the first all-sky maps of ∼0.1-6 keV Energetic Neutral Atoms (ENAs) from the boundaries and discovered a smoothly varying, globally distributed ENA flux overlaid by a narrow “ribbon” of significantly enhanced ENA emissions, which is apparently ordered by the external magnetic field in the LISM. Since then, IBEX has completed five more energy-resolved sets of sky maps and discovered small but important time variations, separated the ribbon and globally distributed fluxes, measured spectral shapes, inferred ion source temperatures, and carried out many other observational and theoretical studies. Most recently, the first three years of IBEX ENA data have been collectively analyzed, including improvements such as correction for the time variable cosmic ray background and orbit-by-orbit calculations of the probability of ENAs surviving on their transits from the outer heliosphere. These results show how the ribbon emissions are driven by the latitude-dependent solar wind speed at solar minimum and suggest that some sort of not yet completely understood secondary ENA process is the most likely source of the ribbon. In a second major area of observations - direct measurements of interstellar neutral atoms from the LISM (six papers in a recent special ApJ Supplement) - IBEX measured interstellar H, He, O, and Ne and shows that the heliospheric motion with respect to the LISM is slower and in a somewhat different direction than thought from prior Ulysses observations. Pulling together the pieces of the slower interstellar flow and larger magnetic field indicated by the IBEX ENA observations (and asymmetric Voyager Termination Shock crossings), these observations and supporting theory and modeling strongly indicate that there is currently no Bow Shock ahead of the heliosphere.