Simultaneous Observation of Negatively and Positively Charged Nanograins at Comet 67P/Churyumov-Gerasimenko

Observations near comet 67P by Rosetta show evidence of charged nanograins. Specifically, the ion and electron sensor (IES) observed negative particles with energy/charge (E/q) consistent with charged nanograins. However, theory predicts that both polarities should be present. On 19 September 2014, IES detected positively and negatively charged particles with E/q up to the maximum detectable by IES (~17 keV/q) coming roughly from the Sun. The arrival directions of the negative and positive particles were different and consistent with their different charges. This observation is the first simultaneous measurement of positively and negatively charged nanograins by a plasma detector in a cometary environment. As observed previously for negative nanograins coming from the comet, we find similar peaks for the picked up negative nanograins, but only one peak for the positive ones. These E/q peaks are interpreted as different charge states since they occur at discrete multiples (1:3 in this event).