CME impact on comet 67P/Churyumov-Gerasimenko

TL;DR

This study reports on Rosetta's observations of the plasma and magnetic field responses at comet 67P during a CME impact, revealing significant compression, increased plasma density, and magnetic flux ropes, advancing understanding of space weather effects on comets.

Contribution

First detailed in-situ analysis of a CME impact on comet 67P, highlighting plasma compression, magnetic flux ropes, and potential reconnection processes.

Findings

Plasma density increased by a factor of 10 during CME impact.

Magnetic field spikes above 200 nT suggest magnetic flux ropes.

Rosetta observed enhanced suprathermal electron flux and magnetic field compression.

Abstract

We present Rosetta observations from comet 67P/Churyumov-Gerasimenko during the impact of a coronal mass ejection (CME). The CME impacted on 5-6 Oct 2015, when Rosetta was about 800 km from the comet nucleus, \textcolor{black}{and 1.4 AU from the Sun}. Upon impact, the plasma environment is compressed to the level that solar wind ions, not seen a few days earlier when at 1500 km, now reach Rosetta. In response to the compression, the flux of suprathermal electrons increases by a factor of 5-10 and the background magnetic field strength increases by a factor of $\sim$2.5. The plasma density increases by a factor of 10 and reaches 600 cm$^{-3}$, due to increased particle impact ionisation, charge exchange and the adiabatic compression of the plasma environment. We also observe unprecedentedly large magnetic field spikes at 800 km, reaching above 200 nT, which are interpreted as magnetic flux ropes. We suggest that these could possibly be formed by magnetic reconnection processes in the coma as the magnetic field across the CME changes polarity, or as a consequence of strong shears causing Kelvin-Helmholtz instabilities in the plasma flow. Due to the \textcolor{black}{limited orbit of Rosetta}, we are not able to observe if a tail disconnection occurs during the CME impact, which could be expected based on previous remote observations of other CME-comet interactions.