mostly located many “magnetic domains” away, an isotropic diffusion should provide at least a reasonable effective description of the data [@BerezinskiiEtAl1990]. For discussions on anisotropic models, see  Refs. [@DeMarcoEtAl2007; @EvoliEtAl2012a].

On the theory side, it is expected that the magnetic turbulence responsible for CR diffusion has different scaling behaviors in $k$-space, as a consequence of various possible phenomena. For instance, the turbulence power spectrum can be dominated by different sources depending on the dynamical range, with the resulting “two-zone” models known to provide good fits to the data [@Tomassetti2012; @GuoEtAl2018] (see also [@SeoEtAl1994; @JonesEtAl2001]). A very appealing scenario is proposed in Refs. [@BlasiEtAl2012a; @AloisioEtAl2013; @AloisioEtAl2015; @EvoliEtAl2018], relying on streaming instability [@Wentzel1969; @Holmes1975; @Skilling1975], where at rigidities beyond a few hundreds of GV, CRs diffuse on the turbulence injected on large scales by supernova bubbles. This turbulence cascades down until crossing the rigidity scale where the turbulence induced by CRs themselves takes over. This naturally generates a break like the one observed in the CR spectra and discussed just above. On the other hand, it is known that the CR spectra observed at low rigidity by the Voyager I spacecraft [@CummingsEtAl2016; @WebberEtAl2017] have a spectral slope rather different from the slope 