Rosetta followed comet 67P from a state of low outgassing from the comet nucleus (low “activity”) at the rendez-vous taking place at 3.6 au to a much higher activity during perihelion at 1.25 au. Initially the solar wind permeated all of the comet atmosphere. As constituents of the comet atmosphere got ionised they are affected by the electric field of the solar wind and accelerated along it. This is initially seen as a deflection of the solar wind in the direction opposite to the solar wind electric field, in accordance with conservation of momentum. As more plasma is added to the solar wind, a process known as “mass-loading”, the solar wind is more and more deflected. Observational results from Rosetta show how this progresses until the solar wind ions gyrate in the mass-loaded plasma around the comet to the extent that a solar wind cavity is formed, as well as a region of enhanced solar wind densities at the flank of the comet. This enhancement resembles the bow waves predicted by some fluid models, but is rather a caustic, it is not connected to a shock. It occurs as a geometric phenomena due to the spatially varying mass-laoding in the cometary coma. A simple model illustrates this (see attached figure). Eventually, the comet atmosphere expands further as does the degree of ionisation. Rosetta spent a large time within the solar wind cavity. It is believed that a more traditional bow shock formed once the comet ion environment became large compared to the cometary ion gyroradius, but Rosetta was not far enough from the nucleus to observe that. Analysis of accelerated cometary ions does however indicate that the cometary ions flowing back towards the nucleus has experienced significant heating to about 10 keV energy somewhere upstream of the nucleus. A simple model of the electric field in the near comet environment indicates that this happened about 4000 km upstream of the comet nucleus, whereas the far dayside excursion of Rosetta only reached a distance of 1500 km. Also closer to the nucleus there are some observations of a broadening of the solar wind energy spectra indicating a shock like heating of the solar wind. This can however happen within the solar wind as a high speed stream overtakes a slower solar wind speed. With better understanding of the comet ion environment we now think that we can identify some of these structures as shocks or shock-lets, the first precursors of the typical bow shock observed around planets and active comets. We suggest a terminology where the comet - solar wind interaction goes from kinetic to fluid-like. Another interesting feature observed closer to the nucleus is a diamagnetic cavity, a region free not only of solar wind ions but also of any significant magnetic field. Initial attempts to find any pressure balance at this boundary has failed. The idea based on data from observations of comet Halley, that friction between the solar wind and the expanding neutral atmosphere causes this boundary, does not hold for comet 67P. We briefly mention some ideas on how this likely is a collisionless phenomena.
PJ, lecture hall, Fysik Origo, Fysik