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 first characterization of cometary electrons with kappa distributions. Two three‐dimensional kappa functions were fit to the observations, which we interpret as two populations of dense and warm (density = 10 cm−3, temperature = 2 × 105 K, invariant kappa index = 10−>1000), and rarefied and hot (density = 0.005 cm−3, temperature = 5 × 105 K, invariant kappa index = 1–10) electrons. We fit the observations on 30 October 2014 when Rosetta was 20 km from 67P, and 3 AU from the Sun. We repeated the analysis on 15 August 2015 when Rosetta was 300 km from the comet and 1.3 AU from the Sun. Comparing the measurements on both days gives the first comparison of the cometary electron environment between a nearly inactive comet far from the Sun and an active comet near perihelion. We find 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 find 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.

Statistical analysis of suprathermal electron drivers at 67P/Churyumov–Gerasimenko

“…However, Broiles et al (2016) found that laboratory testing could not account for all possible distortions, including spacecraft blockages, differential sensitivity between detectors, or the ageing of spacecraft components in flight. They developed a new version of the geometric factor for the electron sensor based observations made in flight, which assumes that the electron distribution is isotropic.…”

“…Consequently, Clark et al (2015) concluded that the electrons must be heated by waves, and speculated that lower hybrid waves were a likely candidate. Broiles et al (2016) extended this study of the cometary electron environment at 67P by performing case studies when it was far from the Sun (∼3 au), and near perihelion (∼1.3 au). They also found that the velocity space density of cometary electrons was well fitted by the summation of two kappa functions.…”

“…They also found that the velocity space density of cometary electrons was well fitted by the summation of two kappa functions. Broiles et al (2016) characterized the two kappa functions as a warm and dense population, and a hot and rarefied population. They concluded that they were observing the mid and hot populations described by Zwickl et al (1986) for the flyby of Giacobini-Zinner.…”

“…In this work, we attempt to determine the origin of the warm electron population described by Broiles et al (2016) using statistical analysis of the warm electron population, neutral gas, and magnetic field surrounding comet 67P/Churyumov-Gerasimenko. We rely on the same fitting and instrument calibration techniques as defined by Broiles et al (2016) to infer the physical parameters of the warm electron population on 2014 November 1.…”

“…We rely on the same fitting and instrument calibration techniques as defined by Broiles et al (2016) to infer the physical parameters of the warm electron population on 2014 November 1. We then compare the warm electron density and temperature with the magnetic field strength and neutral gas density at Rosetta.…”