Ratio of the nonthermal radio (~2.64 GHz) to thermal infrared (~60μm) surface brightness, normalized for NGC 4449 adopting the values of Chyży et al. (2011), as a function of the star formation surface density. We assume a size of the star-forming region of 0.5 kpc, and explore values for the disk height of 0.1,0.25,and0.5 kpc, which influence the critical star formation surface density concerning the relevance of cosmic-ray diffusion losses (Eq. (32)). The transition illustrated here is relevant in the regime of slow rotation defined by Eq. (34), where cosmic-ray diffusion effects become relevant before the relation between magnetic field strengths and star formation surface density breaks down.