Surface angular velocity as a function of time in test calculations of our 1.5 M_⊙ magneto-rotational main-sequence model, in which constant radial magnetic field of B_r = 1 kG and a step function with an arbitrary amplitude for the initial angular momentum distribution are imposed as initial conditions. The star starts to oscillate on the Alfvén time scale. Lines correspond to the case without dissipation (red, solid), including only hydrodynamic instabilities (green, dashed), and the case with hydrodynamic and magneto-hydrodynamic instabilities (blue, dash-dotted), respectively. The wave-crossing time of t_wc = 1.71 × 10⁹ s is referenced by the thick black bar at the bottom left corner.