Observations of counter-streaming ions in and around the diamagnetic cavity of comet 67P

Ion speeds at comet 67P/Churyumov-Gerasimenko significantly exceed local neutral gas speeds, a discrepancy that challenges current ion-neutral coupling models. The presence of counter-streaming ion populations is a proposed solution, as it can reconcile high individual velocities with a low bulk flow speed and potentially drive wave activity through ion-ion instabilities. We aim to identify and statistically characterize counter-streaming ion populations to provide the first quantitative constraints on this phenomenon. We analyzed high- and standard-resolution data from the Ion Composition Analyser (ICA), part of the Rosetta Plasma Consortium (RPC) on board the Rosetta spacecraft. The events were selected from periods when Rosetta was inside or near the diamagnetic cavity, a key region for cometary plasma dynamics. A total of 112 events are identified in the high-resolution data and 338 in the standard-resolution data, both inside and outside the diamagnetic cavity. In general, the anticometward ion population exhibits higher energy and particle counts compared to the cometward ion population. The angular separation between the two flows is peaked in the range of 150^̧irc--180^̧irc. The superposition of these high-speed flows produce net bulk velocities of a few km/s. Our findings provide statistical evidence of counter-streaming ions at a comet and demonstrate that they can lead to a low net bulk speed. Return flows can be explained by gyration of the ions in the magnetic field just outside the diamagnetic cavity.