Ionospheric plasma of comet 67P probed by Rosetta at 3 AU from the Sun

Abstract

We propose to identify the main sources of ionization of the plasma in the coma of comet 67P/Churyumov–Gerasimenko at different locations in the coma and to quantify their relative importance, for the first time, for close cometocentric distances (<20 km) and large heliocentric distances (>3 au). The ionospheric model proposed is used as an organizing element of a multi-instrument data set from the Rosetta Plasma Consortium (RPC) plasma and particle sensors, from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis and from the Microwave Instrument on the Rosetta Orbiter, all on board the ESA/Rosetta spacecraft. The calculated ionospheric density driven by Rosetta observations is compared to the RPC-Langmuir Probe and RPC-Mutual Impedance Probe electron density. The main cometary plasma sources identified are photoionization of solar extreme ultraviolet (EUV) radiation and energetic electron-impact ionization. Over the northern, summer hemisphere, the solar EUV radiation is found to drive the electron density – with occasional periods when energetic electrons are also significant. Over the southern, winter hemisphere, photoionization alone cannot explain the observed electron density, which reaches sometimes higher values than over the summer hemisphere; electron-impact ionization has to be taken into account. The bulk of the electron population is warm with temperature of the order of 7–10 eV. For increased neutral densities, we show evidence of partial energy degradation of the hot electron energy tail and cooling of the full electron population