The Lyman-α Sky Background as Observed by New Horizons

Abstract

Recent observations of interplanetary medium (IPM) atomic hydrogen Lyman-α (Lyα) 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 Lyα 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.

arXiv comment21 pages, 5 figures, 1 table, accepted for publication in Geophys. Res. Lett

References in corpus (5)

• Structure and Composition of Pluto's atmosphere from the New Horizons Solar Ultraviolet Occultation
• Solar parameters for modeling interplanetary background
• The Interstellar H Flow: Updated Analysis of SOHO/SWAN Data
• UV photolysis, organic molecules in young disks, and the origin of meteoritic amino acids
• Voyager 1/UVS Lyman $\alpha$ measurements at the distant heliosphere (90-130AU): unknown source of additional emission

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• Influence of Heliolatitudinal Anisotropy of Solar FUV/EUV Emissions on Lyman-alpha Helioglow: SOHO/SWAN Observations and WawHelioGlow Modeling
• Excess Ultraviolet Emission at High Galactic Latitudes: A New Horizons View
• The Lyman alpha Sky as Observed by New Horizons
• Seasonal variation of Saturn's Lyman-$\alpha$ brightness