Illustration of the Walén relation across a reconnection site. Magnetic field lines are drawn in black, while white arrows represent the plasma flow in the de Hoffman-Teller frame (de Hoffmann & Teller 1950). The rotational discontinuities delimiting the exhaust are shown as dashed lines, with their colour indicating which sign of the Walén relation applies at this boundary. An illustrative spacecraft trajectory is plotted as a dotted arrow. Figure adapted from Gosling et al. (2005a).