Models of interstellar cosmic rays and their propagation into a cloud. Left: the spectrum of interstellar CR protons that are impinging on the cloud at its boundary (N_H2 = 0). For energies E ≳ 1 GeV, the spectrum is constrained by AMS02 observations (Aguilar et al. 2015). At lower energies, the spectrum is constrain by Voyager observations (Cummings et al. 2016; Stone et al. 2019), however, since these observations are likely not probing a representative interstellar spectrum (see Sect. 3.2), we consider different models with different low-energy spectral slopes, α. Right: the resulting CR ionization and excitation rates inside the cloud as a function of α and the clouds’ molecular column density N_H2, as calculated by our CR propagation model.