Ratio τ_disc/τ_DRΔx² is plotted as a function of kinetic energy E_k for three different configurations. In all cases, the parameters of Model A have been assumed with the exception of the Alfvénic speed. The solid black curve corresponds to the critical value V_a = 1.75 km s⁻¹ above which the instabilities which affect the numerical solution u in Fig. D.5 are noticeably reduced. For that velocity, the ratio τ_disc/τ_DRΔx² reaches a maximum of 0.5. Enlarging the range of Alfvénic speeds for which the onset of stability occurs yields the yellow band. The long dashed red and short dashed-dotted purple curves both correspond to V_a = 0.5 km s⁻¹. In the former case, the spacing N_dec of the energy grid is 25, whereas it is 300 in the latter.