Development of a cometosheath at comet 67P/Churyumov-Gerasimenko

H N Williamson,C Goetz,H Nilsson,A Moeslinger,G Stenberg Wieser

doi:10.1051/0004-6361/202142461

Abstract

Context. The ionosphere of a comet is known to deflect the solar wind through mass loading, but the interaction is dependent on cometary activity. We investigate the details of this process at comet 67P using the Rosetta Ion Composition Analyzer. Aims. This study aims to compare the interaction of the solar wind and cometary ions during two different time periods in the Rosetta mission. Methods. We compared both the integrated ion moments (density, velocity, and momentum flux) and the velocity distribution functions for two days, four months apart. The velocity distribution functions were projected into a coordinate system dependent on the magnetic field direction and averaged over three hours. Results. The first case shows highly scattered H+ in both ion moments and velocity distribution function. The He2+ ions are somewhat scattered, but less so, and appear more like those of H2O+ pickup ions. The second case shows characteristic evidence of mass-loading, where the solar wind species are deflected, but the velocity distribution function is not significantly changed. Conclusions. The distributions of H+ in the first case, when compared to He2+ and H2O+ pickup ions, are indicative of a narrow cometosheath on the scale of the H+ gyroradius. Thus, He2+ and H2O+, with larger gyroradii, are largely able to pass through this cometosheath. An examination of the momentum flux tensor suggests that all species in the first case have a significant non-gyrotropic momentum flux component that is higher than that of the second mass-loaded case. Mass loading is not a sufficient explanation for the distribution functions and momentum flux tensor in the first case, and so we assume this is evidence of bow shock formation.

Highlights

Ion distributions for cometary and solar wind plasmas have been measured at a number of comets, including a flyby of comet 21P/Giacobini-Zinner by the International Cometary Explorer (ICE) mission in 1985 (Brandt et al 1985), a comet 1P/Halley flyby by the Giotto spacecraft in 1986 (Coates et al 1989; Huddleston et al 1993; Neugebauer et al 1989), and a subsequent Giotto flyby of comet 26P/Grigg-Skjellerup in 1992 (Coates et al 1993; Grensemann & Schwehm 1993)
The magnetic pressure is greater than the solar wind momentum flux, indicating an increase in the magnetic field
Nongyrotropy Because the distribution functions for the solar wind species in case 1 are indicative of time-varying nongyrotropic effects, we examined the diagonal and non-diagonal elements of the momentum flux tensor

Summary

Introduction

Ion distributions for cometary and solar wind plasmas have been measured at a number of comets, including a flyby of comet 21P/Giacobini-Zinner by the International Cometary Explorer (ICE) mission in 1985 (Brandt et al 1985), a comet 1P/Halley flyby by the Giotto spacecraft in 1986 (Coates et al 1989; Huddleston et al 1993; Neugebauer et al 1989), and a subsequent Giotto flyby of comet 26P/Grigg-Skjellerup in 1992 (Coates et al 1993; Grensemann & Schwehm 1993). The Rosetta mission provided unique, long-term observations of comet 67P/Churyumov-Gerasimenko through perihelion, following it from 2014 to 2016. The size of a cometary ionosphere changes with cometary activity, which varies from comet to comet and within a single comet’s orbit. Outgassing at a comet is largely driven by the amount of insolation; comet 67P increased in activity with decreasing heliocentric distance (Hansen et al 2016; Nilsson et al 2017). The long duration of the Rosetta mission presents an opportunity to study the interaction of a comet with the solar wind through varying levels of activity and heliocentric distances, unlike previous flyby missions.

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