Time evolution of the Schwarzschild monopole (Ω_F = Ω_BH/2) of a slowly spinning Kerr BH (a^* = 0.1, M = 1) in Cartesian (3D CARPET grid with nine refinement levels, with the highest resolution of 0.03125 M_⊙ completely enclosing the central object) and spherical (2D, axsymmetric) coordinates. The spacetime metric is dynamically evolving. Left: evolution of the total magnetospheric (electromagnetic) energy normalized to the initial value, e₀. Right: field line angular velocity along the equatorial plane. The final value is shown in a strong color. Intermediate states throughout the simulation are depicted by lighter colored lines (the strength of the color increasing with simulation time).