RPC-MIP observations at comet 67P/Churyumov-Gerasimenko explained by a model including a sheath and two populations of electrons

G Wattieaux,P Henri,L Bucciantini,N Gilet,X Vallières

doi:10.1051/0004-6361/201834872

Abstract

The response of the mutual impedance probe RPC-MIP on board Rosetta orbiter electrostatically modeled considering an unmagnetized and collisionless plasma with two Maxwellian electron populations. A vacuum sheath surrounding the probe was considered in our model in order to take the ion sheath into account that is located around the probe, which is immersed in the cometary plasma. For the first time, the simulated results are consistent with the data collected around comet 67P/Churyumov-Gerasimenko (67P), but strong discrepancies were identified with the previous simulations that neglected the plasma sheath around the probe. We studied the influence of the sheath thickness and of the electron populations. This work helps to better understand the initially unexpected responses of the mutual impedance probe that were acquired during the Rosetta mission. It suggests that two electron populations exist in the cometary plasma of 67P.

Highlights

Intended for geological and archaeological prospection, mutual impedance probes have been adapted (Storey et al 1969) and successfully used in space plasma investigations since the 1970s (Beghin & Debrie 1972; Décréau et al 1978; Beghin et al 1982; Bahnsen et al 1986; Trotignon et al 2007), and it is planned to use them for future space missions such as BepiColombo to the planet Mercury (Trotignon et al 2006) or the Jupiter icy moons explorer (JUICE)
It was suspected that the problem came from the ion sheath between the probe and the plasma, which is caused by the mobility difference between electrons and positive ions in the cometary plasma; this had been neglected in the modeling
With the insight brought by our electrostatic modeling of the mutual impedance probe RPC-MIP on board the Rosetta orbiter, the initially unexpected experimental responses observed around comet 67P can be related to the plasma evdf and to the sheath thickness surrounding the probe

Summary

Introduction

Intended for geological and archaeological prospection, mutual impedance probes have been adapted (Storey et al 1969) and successfully used in space plasma investigations since the 1970s (Beghin & Debrie 1972; Décréau et al 1978; Beghin et al 1982; Bahnsen et al 1986; Trotignon et al 2007), and it is planned to use them for future space missions such as BepiColombo to the planet Mercury (Trotignon et al 2006) or the Jupiter icy moons explorer (JUICE) This active electrostatic probe introduces a sine wave electrical current with a constant amplitude in the probe surroundings, and it measures the induced electrical potential drop between two receiving electrodes (Storey 1998). Because the sheath thickness is usually in the same order of magnitude as the Debye length, a study of the electrostatic influence of a large sheath

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Discussion

Conclusion