Energetic Neutral Atom Emission from the Heliospheric Nose

The bright energetic neutral atom (ENA) emission associated with the Interstellar Boundary Exporer (IBEX) ribbon has largely obscured the identification, and thus analysis and interpretation, of underlying ENA emission from the general region of the heliospheric nose, or upwind direction. Recent observations of a dramatic ENA emission intensification centered south of the nose region in response to a sustained pressure increase in the solar wind (McComas et al., 2018) has allowed clear identification of non-ribbon emission from this region. In this study, we exploit systematic variations in spatial, spectral, and temporal ENA measurements in the energy range 0.7 to 4.3 keV over the first eight years of IBEX ENA sky maps (before the abrupt ENA intensification in the nose region in 2017) to uniquely reveal the ENA emission from the nose region. This emission is diffuse and generally centered near ecliptic latitude -15°. Above 2 keV the emission is centered directly south of the nose. However, at 0.7 and 1.1 keV, the diffuse nose emission is centered south, but at higher ecliptic longitudes than both the nose direction and ribbon center, suggesting an influence of the interstellar magnetic field on lower energy ENA emission near the nose. This is consistent with the bulk pressure calculated from IBEX flux maps derived after empirical subtraction of the modeled Gaussian-shaped ribbon flux (Schwadron et al., 2014) and consistent with Voyager 2 observations of plasma flow along the pressure gradient (McComas and Schwadron, 2014). While the ribbon flux in the vicinity of the nose region decreases over the eight years of this study, ENA emission from the nose region is generally constant over this time and has a softer spectrum in comparison to the ribbon emission, suggesting that the upstream interaction is largely moderated by the solar wind pickup ion distribution.

Schwadron, et al. (2014), Astrophys. J. Suppl. Ser., 215:13, doi: 10.1088/0067-0049/215/1/13

McComas & Schwadron (2014), Astrophys. J., 795:L17, doi: 10.1088/2041-8205/795/1/L17

McComas, et al.(2018), Astrophys. J., 856:L10, doi: 10.3847/2041-8213/aab611