Impact of a cometary outburst on its ionosphere

Rosetta Plasma Consortium observations of the outburst exhibited by comet 67P/Churyumov-Gerasimenko on 19 February 2016

¹ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace (LPC2E), CNRS, Orléans, France
e-mail: rajkumar.hajra@cnrs-orleans.fr, rajkumarhajra@yahoo.co.in
² Imperial College, South Kensington Campus, London SW7 2AZ, UK
³ Institutet för rymdfysik, Ångström Laboratory, Lagerhyddsvagen 1, 751 05 Uppsala, Sweden
⁴ Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510, USA
⁵ Institut für Geophysik und extraterrestrische Physik, TU Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
⁶ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁷ Swedish Institute of Space Physics, PO Box 812, 981 28 Kiruna, Sweden
⁸ Physikalisches Institut, Universität Bern, Sidlerstr. 5, 3012 Bern, Switzerland

Received: 10 February 2017
Accepted: 27 June 2017

Abstract

We present a detailed study of the cometary ionospheric response to a cometary brightness outburst 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 an increase in the coma surface brightness of two orders of magnitude. 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 outburst, the neutral density increased by a factor of ~1.8 and the local plasma density increased by a factor of ~3, driving the spacecraft potential more negative. The energetic electrons (tens of eV) exhibited decreases in the flux of factors of ~2 to 9, depending on the energy of the electrons. The local magnetic field exhibited a slight increase in amplitude (~5 nT) and an abrupt rotation (~36.4°) in response to the outburst. A weakening of 10–100 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.