Development of a cometosheath at comet 67P/Churuyumov-Gerasimenko: A case study comparison of Rosetta observations

TL;DR

This study compares two different interaction regimes of the solar wind with comet 67P, revealing the formation of a narrow cometosheath and evidence of bow shock formation based on ion distribution and momentum flux analysis.

Contribution

It provides a detailed comparison of ion interactions at comet 67P during different activity levels, highlighting the conditions for bow shock formation and the structure of the cometosheath.

Findings

Highly scattered H+ ions indicating a narrow cometosheath

Mass loading causes deflection but not significant distribution change

Evidence of bow shock formation in the first case

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.