Solar wind observations at comet 67P around perihelion. Initial transition to a fluid-like flow

Rosetta followed comet 67P at heliocentric distances from 1.25 to 3.6 au. Close to perihelion, Rosetta was located in the solar wind ion cavity, with only sporadic observations of solar wind ions. Just outside the solar wind ion cavity, the solar wind ion-flow direction was mainly sunward. We aim to study the evolution of solar wind interaction with a comet as its scale size increases from sub-ion gyroradius to above the proton gyroradius. The former was observed at 67P during most of the Rosetta mission. The latter was observed during sporadic solar wind encounters around perihelion. We analysed particle data from the mass-resolving ion spectrometer ICA, searching for the weak and sporadic occurrences of solar wind ions close to perihelion. All Rosetta plasma data were used to understand the plasma parameter regime of the observed interaction. At comet 67P, we observe the transition from solar wind interaction during the first partial gyration of the solar wind ions in the slowed-down plasma around the comet to multiple gyrations in an interaction region larger than the proton gyroradius. During the multiple-gyration stage, the protons show a consistent pattern of motion ordered by the direction of the solar wind electric field. In the hemisphere where the electric field points away from the nucleus, the protons move sunward. They move anti-sunward in the opposite hemisphere. This can be explained by simple trochoid trajectories and a strong density gradient. Under the same conditions, the water ions also show a consistent flow pattern relative to the electric field determined by the point in their gyration at which they pass closest to the nucleus. Protons are slowed down and heated, which is consistent with a shock upstream. The plasma beta is close to one, and the magnetosonic Mach number was typically less than unity.