Electric (blue lines) and magnetic (golden lines) field lines of the Papapetrou-Wald solution in the xz-plane in Cartesian coordinates. The BH spin parameter is set to a/M = 0.3 and the magnetic field and the BH spin are aligned and parallel. The background is a density-plot of the electric field energy density which is decreasing from red to blue. The BH horizon is the black disk. Distances are in units of M and the fields in units of B₀. Outward electron acceleration occurs in the region limited by the dashed black lines, i.e., where the electric field is inwardly directed. In the northern hemisphere it covers spherical polar angles (measured clockwise) −θ_± < θ < θ_±, where θ_± ≈ 55°. By equatorial symmetry, in the southern hemisphere, it covers π − θ_± < θ < π + θ_±.