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Aims. The Alice far-ultraviolet spectrograph onboard Rosetta is designed toobserve emissions from various atomic and molecular species from within thecoma of comet 67P/ Churyumov-Gerasimenko and to determine their spatialdistribution and evolution with time and heliocentric distance. Methods.Following orbit insertion in August 2014, Alice made observations of the innercoma above the limbs of the nucleus of the comet from cometocentric distancesvarying between 10 and 80 km. Depending on the position and orientation of theslit relative to the nucleus, emissions of atomic hydrogen and oxygen wereinitially detected. These emissions are spatially localized close to thenucleus and spatially variable with a strong enhancement above the comet's neckat northern latitudes. Weaker emission from atomic carbon and CO weresubsequently detected. Results. Analysis of the relative line intensitiessuggests photoelectron impact dissociation of H2O vapor as the source of theobserved H I and O I emissions. The electrons are produced by photoionizationof H2O. The observed C I emissions are also attributed to electron impactdissociation, of CO2, and their relative brightness to H I reflects thevariation of CO2 to H2O column abundance in the coma.

Measurements of the Near-Nucleus Coma of Comet 67P/Churyumov-Gerasimenko with the Alice Far-Ultraviolet Spectrograph on Rosetta