Setting a Lower Limit for the Distance to the IBEX ENA Ribbon

Three years have passed since the surprising discovery of a "ribbon" of excess Energetic Neutral Atom (ENA) flux seen in the first maps from the Interstellar Boundary Explorer (IBEX) mission (McComas et al., 2009, doi:10.1126/science.1180906). Yet there is still no consensus on the origin of this ribbon. Most of the theories place the ribbon near the heliopause boundary between the solar wind and the interstellar medium, but origins near the termination shock and far beyond the heliopause at the boundary between the Local Interstellar Cloud and the nearby g-cloud (Grziedzielski et al., 2010, doi: 10.1088/2041-8205/715/2/L84) have also been proposed. Using astronomical parallax to determine the distance to the ribbon is complicated because the ribbon is a diffuse structure and there are time variations in the ENA intensities. Even so, with six completed IBEX all-sky maps, it is possible to set a lower limit for the distance to the IBEX ENA Ribbon.