Observation of dual proton populations by the Rosetta Ion Composition Analyser

During Rosetta’s 2-year observation period of comet 67P/Churyumov-Gerasimenko, the Ion Composition Analyser (ICA) continuously measured the plasma environment around the comet. The interaction of the solar wind with the cometary plasma and the evolution of the observed solar wind over the course of the mission has been subject of previous studies. It usually shows a single proton population with a large anti-sunward component that gets more and more deflected when the comet approaches perihelion. 

In this study we focus on ICA data obtained during the 19th of April 2016, where we detected two clear peaks in the energy spectra of the proton population. For the level of cometary activity during this time period, a few months after perihelion, a deflected single population is characteristic for the solar wind protons. We attempt to separate these two observed proton populations in the mass-separated ICA data. We then analyse selected plasma properties of the two populations, such as flow velocity (magnitude and direction) and temperatures. This dual proton population is sporadically observed throughout the day, but is otherwise uncommon during the mission. We want to study how these occurrences are related to changes in the cometary environment and the interaction with the solar wind.

A previous study has shown that the difference in proton and alpha particle velocity downstream of a shock can be used to estimate the electrostatic potential of the observation point relative to the solar wind. We take a look on how to interpret the electrostatic potential estimate using the newly estimated proton velocities of both populations.