Ratio R of cometary ion density to solar wind density in the x − z plane, where the undisturbed convection electric field lies. A) Photoionisation only. B) Photoionisation + electron ionisation. C) Photoionisation + charge exchange. D) All processes. The cometopause location is seen as the thin white region between the blue (cometary ion-dominated) and red (solar wind-dominated) regions, defined here as the positions where R= 1. The bow shock can be seen as a darker red paraboloid region that corresponds to an increased solar wind density upstream of the cometopause.