Solar wind dynamics around a comet - A 2D semi-analytical kinetic model

TL;DR

This paper develops a 2D semi-analytical kinetic model to describe solar wind proton trajectories around a comet, capturing deflection patterns and particle-free regions consistent with Rosetta mission observations.

Contribution

It introduces a novel semi-analytical approach to model single particle solar wind dynamics near a comet, emphasizing the motional electric field's role.

Findings

Protons experience deflection perpendicular to their velocity with amplitude proportional to 1/r^2.

A particle-free circular region, or ion cavity, is identified around the comet.

Model results align with in situ data from the Rosetta mission.

Abstract

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. 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. 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/r^2. 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. 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.