Dawn-dusk asymmetry induced by the Parker spiral angle in the plasma dynamics around comet 67P/Churyumov-Gerasimenko

TL;DR

This study investigates how the Parker spiral magnetic field influences plasma interactions around comet 67P, revealing dawn-dusk asymmetries in ion drifts and challenging previous planar assumptions.

Contribution

It provides the first analysis linking Parker spiral geometry to plasma asymmetries at a comet, supported by Rosetta data and analytical modeling.

Findings

Pick-up ions drift dawnward, solar wind ions drift duskward.

Ion drifts are independent of heliocentric distance.

Interaction dynamics are three-dimensional, not planar.

Abstract

When interacting, the solar wind and the ionised atmosphere of a comet exchange energy and momentum. Our aim is to understand the influence of the average Parker spiral configuration of the solar wind magnetic field on this interaction. We compare the theoretical expectations of an analytical generalised gyromotion with Rosetta observations at comet 67P/Churyumov-Gerasimenko. A statistical approach allows one to overcome the lack of upstream solar wind measurement. We find that additionally to their acceleration along (for cometary pick-up ions) or against (for solar wind ions) the upstream electric field orientation and sense, the cometary pick-up ions are drifting towards the dawn side of the coma, while the solar wind ions are drifting towards the dusk side of the coma, independent of the heliocentric distance. The dynamics of the interaction is not taking place in a plane, as often assumed in previous works.