Synchrotron spectral index for pure diffusion model with primary low-energy electron injection index 1.6. Synchrotron from primary electrons (upper), secondary leptons (middle) and total (lower). The experimental ranges are based on values from the literature as reviewed in the text, and are only intended to be indicative of the general trend since the measurements cover different sky areas. Data: red: Tartari et al. (2008); blue: Rogers & Bowman (2008); cyan: Roger et al. (1999); black dashed: Giardino et al. (2002); black dotted: Platania et al. (1998); green: Platania et al. (2003); cyan: Kogut et al. (2007); orange full: Miville-Deschênes et al. (2008); orange dashed: Gold et al. (2009); orange dotted: Dunkley et al. (2009); black full: Kogut et al. (2011).