Far-ultraviolet aurora identified at comet 67P/Churyumov-Gerasimenko

Abstract

Having a nucleus darker than charcoal, comets are usually detected from Earth through the emissions from their coma. The coma is an envelope of gas which forms through the sublimation of ices from the nucleus, as the comet gets closer to the Sun. In the far ultraviolet, observations of comae have revealed the presence of atomic hydrogen and oxygen emissions. When observed over large spatial scales as seen from Earth, such emissions are dominated by resonance fluorescence pumped by solar radiation. Here we analyse atomic emissions acquired close to the cometary nucleus by the Rosetta spacecraft. In order to identify their origin, we undertake a quantitative multi-instrument analysis of these emissions by combining coincident neutral gas, electron, and spectroscopic observations together. We establish that the atomic emissions detected from Rosetta around comet 67P/Churyumov-Gerasimenko at large heliocentric distances result from the dissociative excitation of cometary molecules by accelerated solar-wind electrons (and not electrons produced from photo-ionisation of cometary molecules as suggested in past studies). We reveal their auroral nature. Similar to the discrete aurorae at Earth and Mars, this newly-discovered cometary aurora is driven by the interaction of the solar wind with the local environment. We highlight how OI 1356Å could be used as a tracer of solar-wind electron variability. The Rosetta spacecraft escorted comet 67P/Churyumov-Gerasimenko (referred as 67P hereafter) for more than two years 1, 2. Onboard, the Alice ultraviolet imaging spectrograph 3 detected Far UltraViolet (FUV) atomic hydrogen and oxygen emissions 4-7 from the cometary coma. Spec

Multi-instrument analysis of far-ultraviolet aurora in the southern hemisphere of comet 67P/Churyumov-Gerasimenko

“…6). Throughout these cases, we see a sizeable contribution from e + CO 2 (<0.9 R) to the OI1027 brightness that is not seen in the northern hemisphere (Galand et al 2020). This is especially prominent in case 8, with a post-perihelion enhanced CO 2 column density of 8.66 × 10 14 cm −2 (see Fig.…”

“…In this study, we employed an extension of a multi-instrument analysis developed by Galand et al (2020) to model the emissions driven by dissociative excitation of cometary neutrals by electron impact. The analysis brings together distinct datasets from several instruments onboard Rosetta.…”

“…When the analysis is applied to a larger heliocentric distance (1.99 au), they cannot explain the observed FUV emission brightnesses. Galand et al (2020) employed a multi-instrument analysis to combine FUV brightnesses with in situ or remote neutral gas observations and in situ measurements of the suprathermal electron flux. This work focused on the northern, summer hemisphere of comet 67P at large helicoentric distances where H 2 O is the major neutral species, demonstrating that electron impact on H 2 O and O 2 are the dominant sources of emissions.…”

“…This work focused on the northern, summer hemisphere of comet 67P at large helicoentric distances where H 2 O is the major neutral species, demonstrating that electron impact on H 2 O and O 2 are the dominant sources of emissions. Galand et al (2020) concluded that emissions are driven by electrons which have been accelerated on large scales rather than locally heated. Acceleration of solar wind electrons by an ambipolar field is the most likely candidate (Deca et al 2017(Deca et al , 2019, meaning these emissions are auroras, a phenomenon which had not previously been observed at a comet in the FUV.…”

“…There has been no forward modelling to determine whether the FUV emissions in the southern hemisphere are also driven by dissociative excitation of cometary neutrals or to understand which neutral species are key to each emission line. We propose to apply an extension of the multi-instrument analysis of Galand et al (2020) to model the FUV emissions in the southern hemisphere of comet 67P at large heliocentric distances.…”