Ionospheric plasma of comet 67P probed byRosettaat 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.

FUV Spectral Signatures of Molecules and the Evolution of the Gaseous Coma of Comet 67P/Churyumov–Gerasimenko

“…These observations of the sunward limb, made from distances between 10 and 30 km from the comet's nucleus, showed emissions of atomic hydrogen, oxygen, and carbon, spatially localized within a few km of the nucleus and attributed to electron impact dissociation of H 2 O and CO 2 vapor. This interpretation was supported by measurements of suprathermal electrons by the Rosetta Plasma Consortium (RPC) instruments on Rosetta (Clark et al 2015;Galand et al 2016).…”

“…Variability of the observed emissions is thus due to both the coma gas density at a given position relative to the nucleus, but also the plasma environment at that time and position. An example of the correlation between the two can be found from a comparison of the Alice spectra from 2014 October 18-19 (Feldman et al 2015) with electron flux models for the same dates based on RPC-LAP data from Galand et al (2016). The Alice data were taken during offnadir limb stares that coincided with the interval "T2" discussed by Galand et al in Table 1.…”

“…This apparent discrepancy is understood by the fact that both H 2 O and CO 2 were below the detection limit of Alice at these times, Mall et al showing that the H 2 O density measured by ROSINA was much lower than at other points in the orbit of Rosetta. The emissions observed by Alice also depend on the energetic electron flux near the nucleus which, in turn, depends on several variables including the neutral gas density (Galand et al 2016).…”

Hybrid modelling of cometary plasma environments

“…Using RPC-IES, Clark et al (2015) show that the interaction of the comet with the solar wind is much more turbulent than anticipated, emphasising the role of mass loading in the formation of boundaries and distinct energy populations. Electron ionisation in the early coma is discussed by Galand et al (2016) to explain the variations in RPC-LAP and RPC-MIP measurements over the winter hemisphere, which photoionisation alone can not account for. Broiles et al (2016), modelling RPC-IES spectra with kappa distributions, find two cometary electron populations, one dense and warm, the other rarefied and hot, and both with temperatures of the order of 10 5 K. They subsequently compare the evolution of each population at two different points in the mission lifetime, between the nearly inactive comet far from the Sun and the active one near perihelion.…”

“…In the process, the produced photoelectrons may be suprathermal, in turn ionising the local neutral species (Cravens 1991). A detailed 1D model of these aspects can be found in Vigren & Galand (2013) and Galand et al (2016). Collision cooling may also occur, such that a population of cold electrons may arise from higher-energy photoelectrons.…”