The source of electrons at comet 67P

TL;DR

This study investigates the origin of electrons at comet 67P using Rosetta data and a 3D collisional model, revealing that electron-impact ionization by solar wind electrons dominates electron production away from perihelion.

Contribution

It introduces a new dataset of electron-impact ionization frequency at 67P and combines observational data with a kinetic collisional model to analyze electron sources and dynamics.

Findings

Electron-impact ionization dominates electron production away from perihelion.

EII frequency varies by up to three orders of magnitude and is influenced by magnetic field and outgassing.

Secondary electrons from solar wind collisions are the main population in a weakly outgassing comet.

Abstract

We examine the origin of electrons in a weakly outgassing comet, using Rosetta mission data and a 3D collisional model of electrons at a comet. We have calculated a new dataset of electron-impact ionization (EII) frequency throughout the Rosetta escort phase, with measurements of the Rosetta Plasma Consortium's Ion and Electron Sensor (RPC/IES). The EII frequency is evaluated in 15-minute intervals and compared to other Rosetta datasets. Electron-impact ionization is the dominant source of electrons at 67P away from perihelion and is highly variable (by up to three orders of magnitude). Around perihelion, EII is much less variable and less efficient than photoionization at Rosetta. Several drivers of the EII frequency are identified, including magnetic field strength and the outgassing rate. Energetic electrons are correlated to the Rosetta-upstream solar wind potential difference, confirming that the ionizing electrons are solar wind electrons accelerated by an ambipolar field. The collisional test particle model incorporates a spherically symmetric, pure water coma and all the relevant electron-neutral collision processes. Electric and magnetic fields are stationary model inputs, and are computed using a fully-kinetic, collisionless Particle-in-Cell simulation. Collisional electrons are modelled at outgassing rates of $Q=10^{26}$ s$^{-1}$ and $Q=1.5\times10^{27}$ s$^{-1}$. Secondary electrons are the dominant population within a weakly outgassing comet. These are produced by collisions of solar wind electrons with the neutral coma. The implications of large ion flow speed estimates at Rosetta, away from perihelion, are discussed in relation to multi-instrument studies and the new results of the EII frequency obtained in the present study.