Mid-plane CR ionisation rate per H₂, ζ, vs. the surface density Σ (bottom scale) and column density N (top scale) plotted for several proton spectra (at a distance of 1 AU from the central star), as shown in the inset. Galactic CRs with solar mean modulation (dotted line); minimum and maximum modulation by a TT wind (dash-dot-dotted and long dashed line, respectively); and stellar CRs from an active TT star (dash-dotted line). The horizontal dashed line shows the ionisation rate set by LLR. For reference, black triangles on the horizontal axis indicate the values of the mid-plane disc density (Σ_disc ∕2) at 1 AU for the LMP, FU Ori, MMSN, and HMP models (134, 532, 850, and 942 g cm⁻², respectively).