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

Abstract

We use observations from the Ion and Electron Sensor (IES) onboard the Rosetta spacecraft to study the relationship between the cometary suprathermal electrons and the drivers that affect their density and temperature. We fit the IES electron observations with the summation of 2 kappa distributions, which we characterize as a dense and warm population (∼10 cm−3 and ∼16 eV) and a rarefied and hot population (∼0.01 cm−3 and ∼43 eV). The parameters of our fitting technique determine the populations’ density, temperature, and invariant kappa index. We focus our analysis on the warm population to determine its origin by comparing the density and temperature to the neutral density and magnetic field strength. We find that the warm electron population is actually two separate sub-populations: electron distributions with temperatures above 8.6 eV and electron distributions with temperatures below 8.6 eV. The two sub-populations have different relationships between their density and temperature. Moreover, the two sub-populations are affected by different drivers. The hotter sub-population temperature is strongly correlated with neutral density, while the cooler sub-population is unaffected by neutral density and is only weakly correlated to magnetic field strength. We suggest that the population with temperatures above 8.6 eV is being heated by lower hybrid waves driven by counterstreaming solar wind protons and newly-formed, cometary ions created in localized, dense neutral streams. To the best of our knowledge, this represents the first observations of cometary electrons heated through wave-particle interactions.