Impact of a cometary outburst on its ionosphere: Rosetta Plasma Consortium observations of the comet 67P/Churyumov-Gerasimenko outburst on 19 February 2016

Abstract

We present a detailed study of the cometary ionospheric response to a cometary brightness out-burst using in-situ measurements for the first time. The comet 67P/ Churyumov-Gerasimenko (67P) at a heliocentric distance of 2.4 AU from the Sun, exhibited an outburst at ∼1000 UT on 19 February 2016, characterized by two orders of magnitude increase in the coma surface brightness. The Rosetta spacecraft monitored the plasma environment of 67P from a distance of 30 km, orbiting with a relative speed of 0.2 m s -1 The onset of the outburst was preceded by “pre-outburst decreases” in neutral gas density at Rosetta, in local plasma density and in negative spacecraft potential at ∼0950 UT. In response to the outbust, the neutral density increased by a factor of ∼1.8, the local plasma density increased by a factor of ∼3, driving the spacecraft potential more negative. The energetic (10s of eV) electrons exhibited decreases in the flux by factors of ∼2 to 9 depending on the energy of the electrons. The local magnetic field exhibited a slight increase (∼5 nT) in amplitude and an abrupt rotation (∼36.4) in response to the outburst. A weakening of 0–100 mHz magnetic field fluctuations was also noted during the outburst, suggesting alteration of the origin of the wave activity by the outburst. The plasma and magnetic field effects lasted for about 4 h, from ∼1000 UT to 1400 UT. The plasma densities are compared with an ionospheric model. This shows that while photoionization is the main source of electrons, electron-impact ionization and a reduction in the ion outflow velocity need to be accounted for in order to explain the plasma density enhancement near the outburst peak.