Evolution of High-Energy Electron Populations as a Function of Heliocentric Distance
Résumé
A dynamic plasma environment is created around the comet when its expanding atmosphere gets ionized by solar EUV radiation and electron-impact ionization. The characteristics of the near-cometary plasma vary with the cometary activity phase (i.e., heliocentric distance). Plasma instruments onboard the ESA/Rosetta spacecraft monitored the plasma environment near comet 67P/Churyumov-Gerasimenko and provided in situ observations of the plasma that enabled us to improve our understanding of different electron populations, their existence and characteristics in the near-comet environment at different steps of the comet activity cycle. High-energy electrons were observed near comet through out the Rosetta's escorting phase. These electrons are able to ionize cometary neutrals and thus they have a direct impact on the total plasma density around the comet. We have studied how the density and temperature of different energetic electron populations vary with heliocentric distance. The properties of these electron populations are estimated by fitting a double-kappa function to the measured electron phase space distribution. The results are compared with values that have been achieved by directly integrating over the phase space. The density and temperature values of the near-cometary electron populations are compared with the core and halo solar wind electron properties and their possible relationship is discussed. Acceleration and heating mechanisms for the electrons suggested in the literature are discussed and confronted to our results.