Rosetta Alice/VIRTIS observations of the water vapour UV electroglow emissions around comet 67P/Churyumov-Gerasimenko

Abstract

Several UV emission lines of the coma of 67P/Churyumov-Gerasimenko have been observed by Alice/Rosetta before the 67P/CG perihelion. The H and O emissions are mainly produced by impact dissociation of water molecules by suprathermal electrons. In this paper, we explore further the electron dissociative excitation of H2O to produce the UV emissions by using simultaneous observations of water and H Lyman β done by the VIRTIS-H and Alice instruments during four periods of time in 2014 December, 2015 March and 2015 May. We used simple theoretical considerations to link the UV brightness to the water vapour column density. Two cases are studied. In the first case, we assume the suprathermal electron density is decreasing radially as the thermal electron population; in the second case, we assume the suprathermal electron density does not vary radially. The second case seems more consistent with the Rosetta Plasma Consortium Ion and Electron Sensor measurements during 2015 March and May. The efficiency of the electron dissociative excitation of H2O is lower during the three last periods of time studied compared to the first period in 2014 December. The variability of the efficiency of the electron dissociative excitation between the four studied periods is not simply inversely proportional to the square of the comet-Sun distance but is most likely associated with the variability of the suprathermal electron distribution.