Normalised pitch-angle gradient of the electron VDF from Fig. 8b in the frame centred on the phase speed of the quasi-parallel whistler waves. Positive pitch-angle gradients ∂f_N/∂α′ are shown in red and negative pitch-angle gradients in blue. They are calculated separately for each velocity bin from the normalised VDF with respect to the pitch-angle increasing from the +Y-axis (0°), to the –Y-axis (180°). The black semi-circles show the constant energy curves in the wave frame, and the horizontal black lines mark v_∥ = v_cyclo ± 10%. Pink arrows indicate the electron diffusion paths according to the scenario described in Sect. 2.