Spatial distribution of low-energy plasma around comet 67P/CG from Rosetta measurements

TL;DR

This study uses Rosetta measurements to map the structured low-energy plasma distribution around comet 67P, revealing correlations with neutral density, a consistent plasma-to-neutral ratio, and local ionization as the main plasma source.

Contribution

It provides detailed spatial mapping of low-energy plasma around comet 67P and identifies local ionization as the primary source, with observed density patterns and modulation effects.

Findings

Highest plasma density in summer hemisphere and neck region

Plasma-to-neutral density ratio of about 1-2x10^-6 at 10 km

Electron density decreases as 1/r within 260 km from the nucleus

Abstract

We use measurements from the Rosetta plasma consortium (RPC) Langmuir probe (LAP) and mutual impedance probe (MIP) to study the spatial distribution of low-energy plasma in the near-nucleus coma of comet 67P/Churyumov-Gerasimenko. The spatial distribution is highly structured with the highest density in the summer hemisphere and above the region connecting the two main lobes of the comet, i.e. the neck region. There is a clear correlation with the neutral density and the plasma to neutral density ratio is found to be about 1-2x10^-6, at a cometocentric distance of 10 km and at 3.1 AU from the sun. A clear 6.2 h modulation of the plasma is seen as the neck is exposed twice per rotation. The electron density of the collisonless plasma within 260 km from the nucleus falls of with radial distance as about 1/r. The spatial structure indicates that local ionization of neutral gas is the dominant source of low-energy plasma around the comet.