Properties of the Energetic Neutral Atom Emission from the Ribbon

Numerous emission sources of energetic neutral atoms (ENAs) have been identified in the measurements obtained from the Interstellar Boundary Explorer (IBEX) mission. These sources include the heliospheric nose and tail, the globally distributed flux (GDF) that appears to persist across the entire sky, and the circular ribbon that is likely ordered by the interstellar magnetic field direction. However, these emission structures can overlap, introducing ambiguity in properties of the individual sources within these areas. Separating the overlapping emissions typically requires assumptions about their shape (as projected on the sky) as well as the magnitude and spatial variability of their ENA flux. In this study, we exploit the substantial temporal variation of the ribbon emission over first four years of the IBEX mission, and over the energy range 0.7 to 4.3 keV, to separate its ENA flux from overlapping emissions from the nose, tail, and GDF. We use non-negative matrix factorization (NNMF) to distinguish these sources by their distinctly different time variability. We find that the ribbon flux is well-represented as a single emission structure whose ENA flux uniformly changes over time, generally consistent with other studies that incorporate assumptions about the spatial variability of ENA emission in the overlapping regions. In particular, the separation of ribbon ENA emission from its direct overlap with the nose emission and GDF emission in the upwind hemisphere enables a better understanding of its global shape over the five energy passbands; better measures of its circularity, width, and eccentricity; and systematic deviations that may indicate turbulence or an imprint of a distorted interstellar magnetic field around the heliosphere. These parameters are essential for understanding the structure and dynamics of the source of the ribbon emission.