Circularity of the interstellar boundary explorer ribbon of enhanced energetic neutral atom (ENA) flux

As a sharp feature in the sky, the ribbon of enhanced energetic neutral atom (ENA) flux observed by the Interstellar Boundary Explorer (IBEX) mission is a key signature for understanding the interaction of the heliosphere and the interstellar medium through which we are moving. Over five nominal IBEX energy passbands (0.7, 1.1, 1.7, 2.7, and 4.3 keV), the ribbon is extraordinarily circular, with a peak location centered at ecliptic (λ_RC, β_RC) = (219.°2 ± 1.°3, 39.°9 ± 2.°3) and a half cone angle of φ_C = 74.°5 ± 2.°0. A slight elongation of the ribbon, generally perpendicular to the ribbon center-heliospheric nose vector and with eccentricity ∼0.3, is observed over all energies. At 4.3 keV, the ribbon is slightly larger and displaced relative to lower energies. For all ENA energies, a slice of the ribbon flux peak perpendicular to the circular arc is asymmetric and systematically skewed toward the ribbon center. We derive a spatial coherence parameter δ_C ≤ 0.014 that characterizes the spatial uniformity of the ribbon over its extent in the sky and is a key constraint for understanding the underlying processes and structure governing the ribbon ENA emission.