Distance to the Energetic Neutral Hydrogen Source from the Heliotail

Abstract

Abstract The interaction of the solar wind (SW) with the partially ionized interstellar medium forms the heliosphere. As the supersonic SW flows away from the Sun and incorporates pickup ions (PUIs), they are slowed, compressed, and heated at a termination shock, creating an energetic ion population in the inner heliosheath. The neutralization of PUIs in the heliosheath creates energetic neutral atoms (ENAs) at ∼keV energies that travel ballistically and can be observed at 1 au by the Interstellar Boundary Explorer (IBEX). IBEX uses single-pixel cameras to map ENAs from the heliosphere. In this study, we analyze IBEX observations of >1 keV ENAs from the heliotail during 2009–2017. The ENA spectral index maximizes near the ecliptic plane and decreases at higher latitudes, reflecting the latitudinal structure of the SW. We show that the angular spread of this structure can be used to derive the distance at which the observed ENAs originate, i.e., their cooling length. Using Ulysses observations of the SW we determine that the distance from the Sun to the source of ∼1–6 keV ENAs in the heliotail is ≥289 ± 35 au in 2009–2013 and ≥489 ± 56 au in 2014–2017, using the distance to the termination shock in the downwind direction as 160 au based on the analysis of McComas et al. The increase in ENA source distance over time suggests that IBEX is observing a fast/hotter plasma parcel propagating down the heliotail before being replaced by slow/cooler plasma as the solar cycle evolves.