Cosmic-ray energy density maps at t = 0 (top left) and t = 0.02 (top right) for an initial angle-dependent sinusoid within a purely circular magnetic field with an Alfvén velocity of 1 and a resolution of 128² cells. The energy is evolved with the streaming advection-diffusion term only. The bottom panel shows the radially averaged energy in theradius interval r = [0.15, 0.35] as a functionof the polar angle θ.