Ultraviolet Observations of Coronal Mass Ejection Impact on Comet 67P/Churyumov–Gerasimenko by Rosetta Alice

John W Noonan,Cyril Simon Wedlund,Lori M Feaga,Rebecca N Schindhelm,Michael A’Hearn,Ronald J Vervack Jr,Thomas Broiles,S Alan Stern,Niklas J T Edberg,Brian A Keeney,Jean-Loup Bertaux,Joel Wm Parker,Andrew J Steffl,Harold A Weaver,Paul D Feldman,Matthew M Knight

doi:10.3847/1538-3881/aac432

Abstract

Abstract The Alice ultraviolet spectrograph on the European Space Agency Rosetta spacecraft observed comet 67P/Churyumov–Gerasimenko in its orbit around the Sun for just over two years. Alice observations taken in 2015 October, two months after perihelion, show large increases in the comet’s Lyβ, O i 1304, O i 1356, and C i 1657 Å atomic emission that initially appeared to indicate gaseous outbursts. However, the Rosetta Plasma Consortium instruments showed a coronal mass ejection (CME) impact at the comet coincident with the emission increases, suggesting that the CME impact may have been the cause of the increased emission. The presence of the semi-forbidden O i 1356 Å emission multiplet is indicative of a substantial increase in dissociative electron impact emission from the coma, suggesting a change in the electron population during the CME impact. The increase in dissociative electron impact could be a result of the interaction between the CME and the coma of 67P or an outburst coincident with the arrival of the CME. The observed dissociative electron impact emission during this period is used to characterize the O2 content of the coma at two peaks during the CME arrival. The mechanism that could cause the relationship between the CME and UV emission brightness is not well constrained, but we present several hypotheses to explain the correlation.

Highlights

The European Space Agency (ESA) Rosetta spacecraft was launched in 2004 to perform an orbital study of the comet 67P/ Churyumov–Gerasimenko, the first mission of its kind
The O I 1356 Å emission peaks nearly simultaneously with the electron density during the coronal mass ejection (CME) arrival and decreases smoothly back to quiescent levels, contrasting the fast drop in electron density measured at the spacecraft after 04:00 UTC (Figure 6)
The electrons detected by Rosetta Plasma Consortium (RPC)–Ion and Electron Spectrometer (IES) are at the spacecraft, whereas the emission of O I 1356 Å may come from anywhere along the line of sight within Alice’s field of view

Summary

Introduction

The European Space Agency (ESA) Rosetta spacecraft was launched in 2004 to perform an orbital study of the comet 67P/ Churyumov–Gerasimenko, the first mission of its kind. Because the RPC measurements are taken in situ and Alice results represent a line-of-sight integration, assumptions must be made about the electron and gas density along the line of sight in order to properly determine this effect’s contribution This hypothesis does not explain the difference in slope between the observed O I 1356 Å emission and the in situ electron measurements, which would be expected to match exactly if the CME electrons were the main contributor due to the short lifetime of the excited state, and under the assumption of uniformity for the CME electron density on the scale of the Rosetta–comet distance.

Results

Discussion

Conclusion