Abstract
Context. The Mutual Impedance Probe (MIP) of the Rosetta Plasma Consortium (RPC) onboard the Rosetta orbiter which was in operation for more than two years, between August 2014 and September 2016 to monitor the electron density in the cometary ionosphere of 67P/Churyumov-Gerasimenko. Based on the resonance principle of the plasma eigenmodes, 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 isotropic electron populations and retrieve their properties. Aims. The goal of this paper is to identify and characterize regions of the cometary ionized environment filled with a mix of cold and warm electron populations, which was observed by Rosetta during the cometary operation phase. Methods. To reach this goal, this study identifies and investigates the in situ mutual impedance spectra dataset of the RPC-MIP instrument that contains the characteristics of a mix of cold and warm electrons, with a special focus on instrumental signatures typical of large cold-to-total electron density ratio (from 60 to 90%), that is, regions strongly dominated by the cold electron component. Results. We show from the observational signatures that the mix of cold and warm cometary electrons strongly depends on the cometary latitude. Indeed, in the southern hemisphere of 67P, where the neutral outgassing activity was higher than in northern hemisphere during post-perihelion, the cold electrons were more abundant, confirming the role of electron-neutral collisions in the cooling of cometary electrons. We also show that the cold electrons are mainly observed outside the nominal electron-neutral collision-dominated region (exobase), where electrons are expected to have cooled down. This which indicates that the cold electrons have been transported outward. Finally, RPC-MIP detected cold electrons far from the perihelion, where the neutral outgassing activity is lower, in regions where no electron exobase was expected to have formed. This suggests that the cometary neutrals provide a more frequent or efficient cooling of the electrons than expected for a radially expanding ionosphere.
Highlights
As a comet nucleus approaches the Sun, the solar thermal forcing increases and the cometary volatiles sublimate so that the outgassing activity of neutral particles increases (e.g., Hansen et al 2016)
This study identifies and investigates the in situ mutual impedance spectra dataset of the Rosetta Plasma Consortium (RPC)-Mutual Impedance Probe (MIP) instrument that contains the characteristics of a mix of cold and warm electrons, with a special focus on instrumental signatures typical of large cold-to-total electron density ratio, that is, regions strongly dominated by the cold electron component
In order to compare the signature of cold electrons by the two instruments, we focus on RPC-MIP Short Debye Length (SDL) phased sub-mode, with simultaneous RPC-LAP measurements, resulting in 124 245 slopes among which 7876 slopes (∼6%) indicated the presence of cold electrons
Summary
As a comet nucleus approaches the Sun, the solar thermal forcing increases and the cometary volatiles sublimate so that the outgassing activity of neutral particles increases (e.g., Hansen et al 2016). A cometary atmosphere, mainly composed of water, carbon monoxide and carbon dioxide (Gasc et al 2017; Hoang et al 2017), expands because of the low cometary gravity. This atmosphere gets ionized through different mechanisms (Cravens et al 1987; Galand et al 2016; Héritier et al 2017, 2018): (i) by photoionisation by extreme ultra violet solar flux, (ii) by electron-impact ionisation by energetic electrons or, (iii) by solar wind impact ionisation and charge exchange, to form a cometary ionosphere that eventually interacts with the surrounding solar wind plasma. Electrons at lower temperatures, resulting from the cooling by neutral-plasma interactions, can be observed (Engelhardt et al 2018). As the comet evolves in the solar system, high-energy electrons (>20 eV) of solar wind origin can be observed around the nucleus (Myllys et al 2019)
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