Characterization of a two-electron temperature plasma in the ionosphere of the comet 67P/Churyumov-Gerasimenko with the mutual impedance experiment (RPC-MIP) of Rosetta

The Mutual Impedance Probe (MIP) of the Rosetta Plasma Consortium (RPC) onboard the Rosetta orbiter operated from August 2014 to September 2016 to measure the electron density in the cometary ionosphere of 67P/Churyumov-Gerasimenko. Recent models of the mutual impedance experiment have shown that in a two-electron temperature plasma, such an instrument is able to separate the two electron populations and retrieve their properties.

Indeed, when the colder electron population dominates the plasma, a double-resonance shapes the acquired mutual impedance spectra that enables to characterize the presence of a mix of a cold and a warm electron population in the surrounded plasma. We have investigated the mutual impedance spectra dataset of the RPC-MIP instrument in order to characterize the presence of a mix of cold and warm electron population during the whole Rosetta cometary operation. Furthermore, through a fitting method between acquired and synthetic mutual impedance spectra taking into account the plasma sheath around the spacecraft and the instrument, it is possible to retrieve the density and the temperature of both electron population.

Focusing on cometary cold electrons, we show that their properties depend on the neutral outgassing activity around the nucleus. We also show that the cold electrons are mainly observed outside the electron-neutral collision-dominated region (exobase), where electrons are expected to have cooled down, indicating that the cold electrons have been transported. Finally, RPC-MIP measured cold electrons far from the perihelion where the neutral outgassing activity is lower, in regions where electron-neutral were not expected to occur, suggesting that the collisional electron cooling is more efficient than previously expected.