New Horizons Cruise Observations of Lyman-α Emissions from the Interplanetary Medium

Abstract

Initial results are presented for observations of interplanetary Lyman-α and Lyman-β emissions in the outer solar system obtained by the Alice ultraviolet spectrograph on the New Horizons spacecraft (the first new such data from outside the orbit of Saturn since the Voyager spacecraft). The observations consist of 6 ∘ ×360 ∘ great-circle swaths on the sky, centered on the ecliptic direction λ = 51. 3 ∘, β = 44. 8 ∘, which passes within ∼ 33 ∘ of the upstream and downstream directions of the interstellar wind. To date, three such scans have been acquired: on October 7, 2007, October 18, 2008, and June 19, 2010 (at which times the New Horizons spacecraft was 7.6, 11.3, and 17.0 AU from the Sun, respectively). The data compare fairly well with model simulations, although the brightness of interplanetary Lyman-α emissions falls off more slowly than expected with radial distance from the Sun. The ratio of Lyman-α/Lyman-β brightnesses in the interplanetary medium agrees well with previous measurements by the Voyager ultraviolet spectrometers.