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

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.