We propose the formation of an International Team to meet at ISSI-Bern in order to address open questions on the plasma environment of comet 67P raised by the recent Rosetta mission. The proposed team is composed of multi-disciplinary experts from 8 countries. It includes Rosetta instrument team members from the Rosetta Plasma Consortium (RPC), Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) and Alice (UV spectrometer), as well as modelers with expertise from MHD and multi-fluid, to hybrid and full kinetic approaches applied at different spatial scales around the comet, which would offer a complete self-consistent modeling approach. The team would also facilitate interactions between observers and modelers, would initiate interaction between plasma and UV observational teams, and would enhance the science return from the Rosetta mission.

We will be focusing on three key, interconnected science topics:

(1) To assess the nature and location of interaction regions between the low-outgassing comet 67P and the solar wind,

(2) To identify the source and main processes affecting the different observed electron populations, from cold to hot,

(3) To evaluate the conditions for ion acceleration and the variation in ion composition.

The ESA/Rosetta spacecraft is the first ever to escort a comet, from 3.6 AU (Aug 2014) to perihelion at 1.24 AU (Aug 2015), and up to 3.8 AU at the end of mission on Sept 30, 2016. The gas, thermal and hot plasma and field sensors measured the properties of the neutral and charged coma, while the UV spectrometer observed the effect of the energetic particles on the coma. The plasma plays a critical role in the interaction of the comet with its space environment; it drives, is influenced by, and attests to the nature of this interaction.

Starting from the Rosetta cometary rendezvous and despite a low outgassing rate at such large distances from the Sun, the plasma was observed to already be dominantly of cometary origin and the bulk of the electrons were unexpectedly warm. As the conditions varied with heliocentric and cometocentric distances, activity and season, the plasma population changed: for instance, the hot electrons (>1 eV) were surprisingly found more intense over the winter hemisphere and plasma density reached at times values larger than those observed over the summer hemisphere. The solar wind-comet interaction also evolved with time and appeared to be of a very different nature from what the Giotto mission revealed at 1P/Halley. Now that the Rosetta mission is over and teams are active in consolidating their dataset, it is very timely (1) to address unanticipated and not yet explained findings from Rosetta – such as the location of the contact surface at large distances and the absence of a bow shock during the dayside excursion; (2) to undertake cross-sensor quantitative analysis of the dataset and interpret it in the light of modeling results.