Variability in the Position of the IBEX Ribbon over Nine Years: More Observational Evidence for a Secondary ENA Source

The ribbon of enhanced energetic neutral atom flux, discovered by the Interstellar Boundary Explorer (IBEX) in 2009, has redefined our understanding of the heliosphere's interaction with the local interstellar medium (LISM). Yet, its origin continues to be a topic of scientific debate. The ribbon is circular and traces the region where the putative LISM magnetic field (B-LISM) is perpendicular to the radial direction from the Sun. Using nine years of IBEX-Hi observations, we investigate the ribbon circularity and location as functions of time and energy. We provide updated locations of the ribbon center at five energy passbands (centered at 0.7, 1.1, 1.7, 2.7, and 4.3 keV) in ecliptic coordinates [longitude, latitude]: [217 degrees.41 +/- 0 degrees.95, 44 degrees.36 +/- 0 degrees.93], [219 degrees.72 +/- 0 degrees.95, 41 degrees.50 +/- 0 degrees.87], [220 degrees.51 +/- 1 degrees.19, 39 degrees.96 +/- 1 degrees.00], [218 degrees.08 +/- 1 degrees.66, 38 degrees.44 +/- 1 degrees.24], and [214 degrees.68 +/- 1 degrees.48, 34 degrees.13 +/- 1 degrees.19] respectively. The weighted mean center location over all energies and all years is [218 degrees.33 +/- 0 degrees.68, 40 degrees.38 +/- 0 degrees.88] and its radius is 74 degrees.81 +/- 0 degrees.65. As viewed by IBEX at 1 au, we find that (1) the ribbon is stable over time, with distinct centers at each energy; (2) ribbon centers exhibit small temporal variations, likely caused by the solar wind (SW) speed and density variations; and (3) ribbon location in the sky appears to be driven by (i) the inherent alignment of the ribbon centers along the plane connecting the presumed BLISM and the heliospheric upwind direction, and (ii) the variable SW structure along the heliographic meridian, further emphasizing that the ribbon source is outside the heliosphere.