Characterizing cometary electrons with kappa distributions

Abstract

The Rosetta spacecraft has escorted comet 67P/Churyumov-Gerasimenko since 6 August 2014 and has offered an unprecedented opportunity to study plasma physics in the coma. We have used this opportunity to make the fi rst characterization of cometary electrons with kappa distributions. Two three-dimensional kappa functions were fi t to the observations, which we interpret as two populations of dense and warm (density=10cm 3 , temperature=2×10 5 K, invariant kappa index=10 > 1000), and rare fi ed and hot (density=0.005cm 3 , temperature=5×10 5 K, invariant kappa index=1 – 10) electrons. We fi t the observations on 30 October 2014 when Rosetta was 20km from 67P, and 3AU from the Sun. We repeated the analysis on 15 August 2015 when Rosetta was 300km from the comet and 1.3AU from the Sun. Comparing the measurements on both days gives the fi rst comparison of the cometary electron environment between a nearly inactive comet far from the Sun and an active comet near perihelion. We fi nd that the warm population density increased by a factor of 3, while the temperature cooled by a factor of 2, and the invariant kappa index was unaffected. We fi nd that the hot population density increased by a factor of 10, while the temperature and invariant kappa index were unchanged. We conclude that the hot population is likely the solar wind halo electrons in the coma. The warm population is likely of cometary origin, but its mechanism for production is not known