Cuts of a 3D alpha VDF in the plane perpendicular to the magnetic field (LH panel) and a plane that contains the magnetic field (RH panel), centred on the fitted bulk speed. The sharp cut-offs in both plots are artificial and due to removal of the proton-dominated areas of the distribution function. The contours are logarithmically spaced. The dashed black lines show the contour at 1/e times the maximum value, which is one thermal width away from the bulk speed, and the red ellipses show the corresponding contour for the fitted bi-Maxwellian.