Example of a possible closed subbeam circulation loop (dashed line) around the magnetic pole (labelled B) that would result in bi-drifting as observed by an observer whose line of sight samples a path indicated by the thick solid line. The rotational axis is somewhere above the magnetic axis (not shown). To observe bi-drifting, the tangents of the path at the intersection points (grey dashed lines) should be roughly parallel, unlike what would occur for a circular path. The X indicates the position of a possible obstruction responsible for the distorted circulation path.