Spatial Distribution of Ultraviolet Emission from Cometary Activity at 67P/Churyumov-Gerasimenko

TL;DR

This study presents the first two-dimensional ultraviolet emission maps of comet 67P/Churyumov-Gerasimenko's coma, revealing spatial variations in dissociative electron impact emissions and O2/H2O ratios during increased activity periods.

Contribution

It introduces a novel UV mapping method for cometary comae, integrating Alice observations with other instruments to analyze spatial emission variations and compositional changes.

Findings

Increased dissociative electron impact emission during activity peaks

O2/H2O ratio approximately 0.3 during outbursts

Spatial extent of emission variations mapped around the nucleus

Abstract

The Alice ultraviolet spectrograph on board the \textit{Rosetta} orbiter provided the first near-nucleus ultraviolet observations of a cometary coma from arrival at comet 67P/Churyumov-Gerasimenko in 2014 August through 2016 September. The characterization of atomic and molecular emissions in the coma revealed the unexpected contribution of dissociative electron impact emission at large heliocentric distances and during some outbursts. This mechanism also proved useful for compositional analysis, and Alice observed many cases that suggested elevated levels of the supervolatile \ce{O2}, identifiable in part to their emissions resulting from dissociative electron impact. In this paper we present the first two-dimensional UV maps constructed from Alice observations of atomic emission from 67P during an increase in cometary activity on 2015 November 7-8. Comparisons to observations of background coma and of an earlier collimated jet are used to describe possible changes to the near-nucleus coma and plasma. To verify the mapping method and place the Alice observations in context, comparisons to images derived from the MIRO and VIRTIS-H instruments are made. The spectra and maps we present show an increase in dissociative electron impact emission and an \ce{O2}/\ce{H2O} ratio of $\sim$0.3 for the activity; these characteristics have been previously identified with cometary outbursts seen in Alice data. Further, UV maps following the increases in activity show the spatial extent and emission variation experienced by the near-nucleus coma, informing future UV observations of comets that lack the same spatial resolution.