PADs of electrons observed when a hypothetical spacecraft crosses an RCS with a single X-nullpoint. The top panel (P) shows that the sampled crossing path of the spacecraft is perpendicular to the RCS midplane (solid green line). The in-plane magnetic field topology is drawn with black lines with the amplitudes of B_y (b_g = 1 case) indicated by the background colours. The PADs of high-energy electrons (second row) and lower-energy electrons (third row) obtained from panel a0 are presented in panels a1 and a2. The PADs shown in panels b1 and b2 are recorded along the same path as in panels a1 and a2 when the virtual spacecraft crosses an RCS with no guiding field (b_g = 1).