Particle flux ratio of He⁺ ions in the symmetric x−yz plane around the comet for event #28 (). The forward analytical model uses Q₀ = Q_S13, v₀ = 700 m s^-1. The Sun is situated at the far right of the figure, with solar wind ions undergoing charge-exchange processes as they move closer to the comet nucleus. The optical shadow of the comet nucleus, assumed to be a sphere of radius r_c for simplicity, is shown in dark blue, corresponding to a negligible column density along the solar wind direction in the wake of the comet. The white line represents Rosetta’s trajectory between 01-01-2015 0UT and 01-02-2015 0UT, while the red circle marks Rosetta’s position for event #28.