Measurements of the electrostatic potential of Rosetta at comet 67P

TL;DR

This study measures and compares the spacecraft potential of Rosetta near comet 67P using multiple instruments, revealing variations driven by cometary activity and instrument limitations, with implications for plasma environment understanding.

Contribution

It provides the first detailed comparison of spacecraft potential measurements from different instruments on Rosetta, highlighting their correlation, offsets, and limitations in a cometary environment.

Findings

Vs/c is mainly negative, driven by high photoelectron temperatures.

Strong correlation between LAP and ICA measurements in many cases.

ICA energy offset of approximately 13.7 eV identified.

Abstract

We present and compare measurements of the spacecraft potential (Vs/c) of the Rosetta space- craft throughout its stay in the inner coma of comet 67P/Churyumov-Gerasimenko, by the Langmuir probe (RPC-LAP) and Ion Composition Analyzer (RPC-ICA) instruments. Vs/c has mainly been negative, driven by the high temperature (~5-10 eV) of the coma photoelectrons. The magnitude of the negative Vs/c traces heliocentric, cometocentric, seasonal and diurnal variations in cometary outgassing, consistent with production at or inside the cometocentric distance of the spacecraft being the dominant source of the observed plasma. LAP only picks up a portion of the full Vs/c since the two probes, mounted on booms of 2.2 and 1.6 m length, respectively, are generally inside the potential field of the spacecraft. Comparing with the minimum energy of positive ions collected by ICA, we find numerous cases with strong cor- relation between the two instruments, from which the fraction of Vs/c picked up by LAP is found to vary between about 0.7 and 1. We also find an ICA energy offset of 13.7 eV (95% CI: [12.5, 15.0]). Many cases of poor correlation between the instruments are also observed, predominantly when local ion production is weak and accelerated ions dominate the flux, or during quiet periods with low dynamic range in Vs/c and consequently low signal-to-noise ratios.