The Lyman-{\alpha} Sky Background as Observed by New Horizons

TL;DR

This paper reports new measurements of interplanetary hydrogen Lyman-alpha emission from New Horizons, confirming previous Voyager results and revealing an additional constant brightness component possibly linked to the hydrogen wall at the heliopause.

Contribution

It provides the first detailed observations of IPM Lyα emission from New Horizons, validating earlier Voyager data and identifying a persistent background brightness.

Findings

NH data agree with Voyager UVS results after brightness correction

Brightness falloff follows a 1/r dependence with a constant offset

Evidence suggests the presence of a hydrogen wall or distant background

Abstract

Recent observations of interplanetary medium (IPM) atomic hydrogen Lyman-{\alpha} (Ly{\alpha}) emission in the outer solar system, made with the Alice ultraviolet spectrograph on New Horizons (NH), are presented. The observations include regularly spaced great-circle scans of the sky and pointed observations near the downstream and upstream flow directions of interstellar H atoms. The NH Alice data agree very well with the much earlier Voyager UVS results, after these are reduced by a factor of 2.4 in brightness, in accordance with recent re-analyses. In particular, the falloff of IPM Ly{\alpha} brightness in the upstream-looking direction as a function of spacecraft distance from the Sun is well-matched by an expected 1/r dependence, but with an added constant brightness of ~40 Rayleighs. This additional brightness is a possible signature of the hydrogen wall at the heliopause or of a more distant background. Ongoing observations are planned at a cadence of roughly twice per year.