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

¹ Department of Physics, Imperial College London, London, SW7 2AZ, UK
e-mail: pps18@ic.ac.uk
² Physics and Astronomy, The Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
³ Department of Physics, Umeå University, 901 87 Umeå, Sweden
⁴ Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
⁵ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
⁶ Southwest Research Institute, Department of Space Studies, Suite 300, 1050 Walnut Street, Boulder, CO 80302, USA
⁷ SouthWest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510, USA
⁸ Swedish Institute of Space Physics, Ångström Laboratory, Lägerhyddsvägen1, 75237 Uppsala, 20 Sweden

Received: 11 August 2020
Accepted: 2 December 2020

Abstract

Aims. We aim to determine whether dissociative excitation of cometary neutrals by electron impact is the major source of far-ultraviolet (FUV) emissions at comet 67P/Churyumov-Gerasimenko in the southern hemisphere at large heliocentric distances, both during quiet conditions and impacts of corotating interaction regions observed in the summer of 2016.

Methods. We combined multiple datasets from the Rosetta mission through a multi-instrument analysis to complete the first forward modelling of FUV emissions in the southern hemisphere of comet 67P and compared modelled brightnesses to observations with the Alice FUV imaging spectrograph. We modelled the brightness of OI1356, OI1304, Lyman-β, CI1657, and CII1335 emissions, which are associated with the dissociation products of the four major neutral species in the coma: CO₂, H₂O, CO, and O₂. The suprathermal electron population was probed by the Ion and Electron Sensor of the Rosetta Plasma Consortium and the neutral column density was constrained by several instruments: the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), the Microwave Instrument for the Rosetta Orbiter and the Visual InfraRed Thermal Imaging Spectrometer.

Results. The modelled and observed brightnesses of the FUV emission lines agree closely when viewing nadir and dissociative excitation by electron impact is shown to be the dominant source of emissions away from perihelion. The CII1335 emissions are shown to be consistent with the volume mixing ratio of CO derived from ROSINA. When viewing the limb during the impacts of corotating interaction regions, the model reproduces brightnesses of OI1356 and CI1657 well, but resonance scattering in the extended coma may contribute significantly to the observed Lyman-β and OI1304 emissions. The correlation between variations in the suprathermal electron flux and the observed FUV line brightnesses when viewing the comet’s limb suggests electrons are accelerated on large scales and that they originate in the solar wind. This means that the FUV emissions are auroral in nature.