Top and middle: schematic illustration of the differences between the thermal (top) and suprathermal (middle) ions regarding CE rates: for a given time interval the number of CE reactions for the two species are proportional to the lengths of the helicoidal curves followed by the ions. The cross-sections do not vary strongly from one species to the other, but the lengths directly depend on the individual speeds that are much higher for the suprathermal species, hence higher CE rates. Bottom: for all types of particles, the smaller the wind bulk speed, the higher the number of charge-exchange reactions, for the same distance traveled by the bulk flow. Because CE reactions with counter-streaming H atoms result in a deceleration of the flow, this constitutes a positive feedback mechanism.