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

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 Ly-$β$, 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.