Electric field measurements at the plasma frequency around comet 67P by RPC-MIP on board Rosetta

¹ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace (LPC2E), CNRS, Orléans, France
e-mail: myllysminna@gmail.com
² Swedish Institute of Space Physics, PO Box 821, 981 28 Kiruna, Sweden
³ Department of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland
⁴ Space Sciences Laboratory, University of California–Berkeley, Berkeley, CA 94720, USA
⁵ CPAESS, University Corporation for Atmospheric Research, Boulder, CO 80301, USA
⁶ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁷ The Centre for Planetary Sciences at University College London/Birkbeck, Gower Street, London WC1E 6BT, UK
⁸ Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510, USA
⁹ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands

Received: 4 September 2019
Accepted: 12 April 2021

Abstract

Context. The Mutual Impedance Probe (RPC-MIP) carried by the Rosetta spacecraft monitored both the plasma density and the electric field in the close environment of comet 67P/Churyumov–Gerasimenko (67P), as the instrument was operating alternatively in two main modes: active and passive. The active mode is used primarily to perform plasma density measurements, while the passive mode enables the instrument to work as a wave analyzer.

Aims. We are reporting electric field emissions at the plasma frequency near comet 67P observed by RPC-MIP passive mode. The electric field emissions are related to Langmuir waves within the cometary ionized environment. In addition, this study gives feedback on the density measurement capability of RPC-MIP in the presence of cold electrons.

Methods. We studied the occurrence rate of the electric field emissions as well as their dependence on solar wind structures like stream interaction regions (SIRs) and coronal mass ejections (CMEs).

Results. We are showing that strong electric field emissions at the plasma frequency near 67P were present sporadically throughout the period when Rosetta was escorting the comet, without being continuous, as the occurrence rate is reported to be of about 1% of all the measured RPC-MIP passive spectra showing strong electric field emissions. The Langmuir wave activity monitored by RPC-MIP showed measurable enhancements during SIR or CME interactions and near perihelion.

Conclusions. According to our results, Langmuir waves are a common feature at 67P during the passage of SIRs. Comparing the plasma frequency given by the RPC-MIP passive mode during Langmuir wave periods with the RPC-MIP active mode observations, we conclude that the measurement accuracy of RPC-MIP depends on the operational submode when the cold electron component dominates the electron density.