The Lyman-α Sky Background as Observed by New Horizons

Recent observations of interplanetary medium atomic hydrogen Lyman-alpha emission in the outer solar system, made with the Alice ultraviolet spectrograph on New Horizons, 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 New Horizons 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 reanalyses. In particular, the falloff of interplanetary medium Lyman-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 similar to 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.