Schematic sketch to explain behaviour of convolution lengthscales as a function of the ratio between ordered and turbulent magnetic field strengths (Fig. 10). Red: Gaussian convolution lengthscales at 6 cm. Blue: Gaussian convolution lengthscales at 20 cm. Grey: exponential convolution lengthscales at 6cm and 20 cm. The grey arrow indicates that the Gaussian convolution lengthscales are about two times larger than the exponential convolution lengthscales (see Sect. 5). The dashed line indicates the transition from diffusion (Gaussian convolution) to streaming (exponential convolution). Thick lines mark the region populated by observed galaxies (cf. Fig. 10).