Measurements of the near-nucleus coma of comet 67P/Churyumov-Gerasimenko with the Alice far-ultraviolet spectrograph on Rosetta

Aims. The Alice far-ultraviolet spectrograph onboard Rosetta is designed to observe emissions from various atomic and molecular species from within the coma of comet 67P/ Churyumov-Gerasimenko and to determine their spatial distribution and evolution with time and heliocentric distance. Methods. Following orbit insertion in August 2014, Alice made observations of the inner coma above the limbs of the nucleus of the comet from cometocentric distances varying between 10 and 80 km. Depending on the position and orientation of the slit relative to the nucleus, emissions of atomic hydrogen and oxygen were initially detected. These emissions are spatially localized close to the nucleus and spatially variable with a strong enhancement above the comet's neck at northern latitudes. Weaker emission from atomic carbon and CO were subsequently detected. Results. Analysis of the relative line intensities suggests photoelectron impact dissociation of H2O vapor as the source of the observed H I and O I emissions. The electrons are produced by photoionization of H2O. The observed C I emissions are also attributed to electron impact dissociation, of CO2, and their relative brightness to H I reflects the variation of CO2 to H2O column abundance in the coma.