Plasma cloud described in the solar wind reference frame (left) and comet reference frame (right). The light grey circle indicates the boundary of the plasma cloud, the central point indicates the direction of the magnetic field off the page. The black horizontal arrows indicate the direction of the plasma cloud velocity in the solar wind reference frame V_i and the solar wind velocity in the comet reference frame V_SW. The arrow denoted E_in × B indicates the direction of the drift of the electrons due to the polarisation electric field shown with a blue arrow. In the right-hand figure we also show the solar wind electric field in the comet reference frame (E_SW) and the net electric field (assuming in the figure that the magnitude of the solar wind and polarisation electric fields are the same). Angle α is the angle from the anti-sunward direction.