CIRCULARITY OF THEINTERSTELLAR BOUNDARY EXPLORERRIBBON OF ENHANCED ENERGETIC NEUTRAL ATOM (ENA) FLUX

Abstract

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.

Effects of solar wind speed on the secondary energetic neutral source of the Interstellar Boundary Explorer ribbon

“…2 we show an example of the simulated ribbon flux rotated to a common ribbon-centered frame. In this frame we derive key geometrical properties, such as the ribbon center, radius, eccentricity, and rotation angle, similar to the analysis of Funsten et al (2013).…”

“…This discrepancy between the simulation and observation is likely due to our use of uniform SW boundary conditions. Also, Funsten et al (2013) found that there is a significant dependence of the IBEX ribbon rotation angle on ENA energy. If the semi-major axis is related (but perpendicular) to the energy-dependent symmetry axis of the IBEX ribbon found by Funsten et al (2015), and the unimodal/bimodal symmetry of the IBEX ribbon as a function of ENA energy is affected by the asymmetric SW, then it is reasonable to suggest that the rotation angle of the IBEX ribbon is also affected by the latitudinal asymmetry of the SW. Non-ribbon ENA emission features may lie within the region scribed by the ribbon and may A31, page 10 of 13 also contribute to both non-circularity and asymmetry at different energies (Funsten et al 2015).…”

“…4), and that the line integration of ENAs masks the differences in the depths of their source. Also, in contrast to the IBEX observations in which the ribbon center was a slight function of ENA energy and the Funsten et al (2013) are shown in filled symbols, with the weighted centers (over energy and fit) shown in black symbols in the top row and bottom-left panel. In the bottom-right panel, the results are weighted over both circular and elliptical fits as well as energy.…”

“…Notes. The IBEX data are from Funsten et al (2013). All results in this table are weighted over energy and circle/ellipse fits.…”

A Ulysses Detection of Secondary Helium Neutrals

“…In Figure 4, we plot in ecliptic coordinates the ISM field direction inferred from the IBEX ribbon center (Funsten et al 2013) and the upwind ISM flow direction from WMW15, with the connecting line indicating the path along a great Funsten et al 2013), the upwind direction of the ISM He flow (green box; WMW15), the upwind direction of the H flow in the solar system (light blue box; Lallement et al 2005Lallement et al , 2010, the IBEX-derived upwind direction of the secondary He flow (solid blue box; Kubiak et al 2016), and the Ulysses-derived upwind direction of the secondary He flow (dotted blue box; this paper). The boxes indicate the quoted uncertainties in the directions.…”