Far Ultraviolet atomic emissions at comet 67P: What have we learned? 

Abstract

Auroral emissions have been observed throughout the Solar System. They are the photo-manifestation of the interaction of energetic, extra-atmospheric particles (typically electrons or ions) with an atmosphere. As the source of energy comes from the space environment (e.g., solar wind or magnetosphere if applicable), the auroral emissions are a tracer of plasma bombardments in an atmosphere. They are also a fingerprint of plasma source and atmospheric species. They are an invaluable, remote-sensing probe of plasma interaction in the Solar System. Through a multi-instrument analysis of gas, particle and spectroscopic dataset from Rosetta, we have established that the atomic emissions observed in the coma of comet 67P at large heliocentric distances (> 2 astronomical units) are of auroral origin [Galand et al., Nature Astronomy, https://doi.org/10.1038/s41550-020-1171-7, 2020; Stephenson et al.., A&A, https://doi.org/10.1051/0004-6361/202039155, 2021]. We will discuss the source of the energetic particles responsible for the Far UltraViolet (FUV) emissions and will highlight the relevance of observing some of them from Earth. We will contrast these emissions with those observed at comets in the soft X-rays and extreme ultraviolet and with the FUV emissions observed at Earth, Mars and Ganymede.