Dominant source of collision velocity in the space of the sizes of the collision partners. The turbulent velocity v_g and the constant c_d, which describes the speed of the ion-neutral drift (see Eq. (5)), are varied between the panels, as labeled. v_g and c_d in each panel take the same values as in the corresponding panel of Fig. 2. The mapping between color and dominant velocity source is shown in the legend on top.