Solar wind dynamics around a comet

Abstract

Aims. We aim at analytically modelling the solar wind proton trajectories during their interaction with a partially ionised cometary atmosphere, not in terms of bulk properties of the flow but in terms of single particle dynamics. Methods. We first derive a generalised gyromotion, in which the electric field is reduced to its motional component. Steady-state is assumed, and simplified models of the cometary density and of the electron fluid are used to express the force experienced by individual solar wind protons during the interaction. Results. A three-dimensional (3D) analytical expression of the gyration of two interacting plasma beams is obtained. Applying it to a comet case, the force on protons is always perpendicular to their velocity and has an amplitude proportional to 1/r2. The solar wind deflection is obtained at any point in space. The resulting picture presents a caustic of intersecting trajectories, and a circular region is found that is completely free of particles. The particles do not lose any kinetic energy and this absence of deceleration, together with the solar wind deflection pattern and the presence of a solar wind ion cavity, is in good agreement with the general results of the Rosetta mission. Conclusions. The qualitative match between the model and the in situ data highlights how dominant the motional electric field is throughout most of the interaction region for the solar wind proton dynamics. The model provides a simple general kinetic description of how momentum is transferred between these two collisionless plasmas. It also shows the potential of this semi-analytical model for a systematic quantitative comparison to the data.

Hybrid modeling of cometary plasma environments

“…Figure A.1 also shows solar wind proton trajectories. For Run 1, we see a similar result to that of Behar et al (2018) and Saillenfest et al (2018), with the convergence of proton trajectories forming an asymmetric caustic and a solar wind cavity. As the coupling between cometary ions and the solar wind increases through increasing IMF magnitude, we see in detail how the M ms = 2 surface is formed, and how finite ion gyroradius effects are visible in the sheath as mild ridges in the Mach number values.…”

“…Our 3D upstream mass-loading extension is based on an approximation that takes the finite gyroradius character of the solution into account and is somewhat similar to the works of Behar et al (2018) and Saillenfest et al (2018). Our approach is based on two approximations in the upstream mass-loading region outside of the nominal simulation box:…”

Solar wind charge exchange in cometary atmospheres

“…Over the entire mission, the deceleration of the solar wind using the mean speed of the particles is much more limited than the deceleration shown by the norm of the bulk velocity : there is more kinetic energy in the solar wind than the bulk velocity vector would let us think. This is the main difference with the paradigm used at previously studied (and more active) comets (Behar et al 2018b). These complex, nonthermal velocity distribution functions also prevent us from reducing the second-order moment (the stress tensor) to a single scalar value, which, for a Maxwellian distribution, could be identified with a plasma temperature.…”

Ionospheric total electron content of comet 67P/Churyumov-Gerasimenko

“…For practical reasons, we take the upper limit of integration H as 500 km. This ensures convergence and is justified by the fact that the cometary plasma density is shown to follow a much steeper variation, closer to r −2 c , at larger distances (see Behar et al 2018) associated with the cometary ion pick-up process in the incoming magnetized solar wind flow; this results in insignificant contribution to cometary ionospheric TEC at large cometocentric distances. It should be noted that this TEC estimation is based on an assumption of radially expanding plasma.…”