Spectrum evolution for the 3D simulation of Alfvén interactions initialized according to Eq. (75). We show the spectral energy distribution for wavenumbers k_⊥ (perpendicular to the guide field) in analogy to Fig. 2, Li et al. (2019). Top: spectral energy distribution at different times for the resolution 384³. Bottom: spectral energy distribution for selected times and wavenumbers (color code as in top panel) and different resolutions, indicated by dashed (128³), dotted (256³) and solid (384³) lines. For wavenumbers k_⊥ ≲ 40, convergence is reached for the shown high-resolution cases.