Abstract
Context. The Rosetta spacecraft escorted comet 67P/Churyumov-Gerasimenko from 2014 to September 2016. The mission provided in situ observations of the cometary plasma during different phases of the cometary activity, which enabled us to better understand its evolution as a function of heliocentric distance. Aims. In this study, different electron populations, called warm and hot, observed by the Ion and Electron Sensor (IES) of the Rosetta Plasma Consortium (RPC) are investigated near the comet during the escorting phase of the Rosetta mission. Methods. The estimates for the suprathermal electron densities and temperatures were extracted using IES electron data by fitting a double-kappa function to the measured velocity distributions. The fitting results were validated using observations from other RPC instruments. We give upgraded estimates for the warm and hot population densities compared to values previously shown in literature. Results. The fitted density and temperature estimates for both electron populations seen by IES are expressed as a function of heliocentric distance to study their evolution with the cometary activity. In addition, we studied the dependence between the electron properties and cometocentric distance. Conclusions. We observed that when the neutral outgassing rate of the nucleus is high (i.e., near perihelion) the suprathermal electrons are well characterized by a double-kappa distribution. In addition, warm and hot populations show a significant dependence with the heliocentric distance. The populations become clearly denser near perihelion while their temperatures are observed to remain almost constant. Moreover, the warm electron population density is shown to be strongly dependent on the radial distance from the comet. Finally, based on our results we reject the hypothesis that hot electron population seen by IES consists of solely suprathermal (halo) solar wind electrons, while we suggest that the hot electron population mainly consists of solar wind thermal electrons that have undergone acceleration near the comet.
Highlights
When a comet travels through interplanetary space and approaches the Sun, its surface temperature increases and volatiles, such as water and carbon dioxide, start sublimating to create an expanding atmosphere around the comet
Escorting comet 67P/Churyumov-Gerasimenko (67P) for more than two years, from 2014 until the end of September 2016, the European Space Agency (ESA)/Rosetta mission offered a unique opportunity to observe the evolution of the cometary plasma environment, as the plasma instruments on board ESA/Rosetta monitored the characteristics of the cometary plasma at different steps of the comet activity cycle
Based on the robust data treatment described above, computed during the whole cometary phase of the Rosetta mission, we concentrate on the properties of the warm and hot electron populations observed at comet 67P
Summary
When a comet travels through interplanetary space and approaches the Sun, its surface temperature increases and volatiles, such as water and carbon dioxide, start sublimating to create an expanding atmosphere around the comet. When such atmosphere gets ionized by solar extreme ultraviolet (EUV) radiation, electron-impact ionization and charge exchange with solar wind ions (Galand et al 2016; Simon Wedlund et al 2016), a cometary ionosphere forms around the comet. The Mutual Impedance Probe (RPC-MIP; Trotignon et al 2007) and two Langmuir probes (RPC-LAP; Eriksson et al 2007) monitored the cometary plasma environment, while the 3D energy distribution of the electrons was measured by the Ion and Electron Sensor (RPC-IES; Burch et al 2007). The lowest energy threshold of RPC-IES was 4.32 eV and even higher when the spacecraft potential was highly negative (Odelstad et al 2015)
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