Velocity dispersion σ_ɀ in the ɀ direction in 20 pc on-a-side cubical bins for the magnetised CHEM_ MHD (blue) and unmagnetised CHEM_HD (red) simulations at R < 500 pc and t = 186 Myr. The simulations are identical except that the one has magnetic fields and the other does not. On the x axis is the average density within the bin. Red and blue solid lines indicate the average σ_ɀ at the given density. Dashed lines indicate the mass-weighted averaged Alſvén speed, and dotted lines the thermal sound speed. The magnetised simulation is significantly more turbulent. The turbulence is driven by the magneto-rotational instability (Sect. 5).