Review of comet 67P/CG ionosphere and its interaction with the solar wind after Rosetta
Résumé
During more than two years, the ESA's Rosetta orbiter has escorted comet 67P/CG, until a final descent down to the comet nucleus in September 2016. This was the first long term in situ survey of the expanding ionosphere of a comet (sometimes referred as an induced magnetosphere) and its interaction with the solar wind. Rosetta has escorted comet 67P/CG at heliocentric distances ranging from 1.2 to 3.8 au, and at various distances from the comet nucleus itself (from 1500 km down to the comet nucleus surface). This has enabled (i) to monitor the close cometary ionised environment associated to large variations of cometary outgassing activity and (ii) to witness the transition between different plasma regimes: from magnetised to unmagnetised, from collisional to collisionless. In this context, I will review the results obtained so far on the ionosphere of comet 67P/CG, based on in situ observations from the different instruments of the Rosetta Plasma Consortium (RPC). In particular, RPC observations have reported the building of a cometary ionosphere, through different ionisation processes, and shown its large scale structure. In the inner cometary ionosphere, the interaction between the ionospheric plasma and the neutral gas has been inferred. In the outer cometary ionosphere, the interaction of the cometary ionosphere with the surrounding magnetised solar wind (induced magnetosphere) has been observed to expel, first, the solar wind ions, and second, the interplanetary magnetic field, therefore building different cometary plasma boundaries and transition regions witnessed by Rosetta. Far from showing a steady-state situation, RPC observations have witnessed during most of the mission a very dynamical, inhomogeneous cometary ionosphere, often embedded in the solar wind, where available free energy is the source for various waves and instabilities to develop. On top of that, the RPC has reported the effects of transient, energetic solar wind (e.g. CIRs and CMEs) or cometary (e.g. outbursts) events on the cometary ionosphere. Finally, I will briefly review the needs in term of cometary plasma modelling to answer some open questions risen after two years of in situ Rosetta measurements of comet 67P/CG ionosphere.