Differential contribution to the ionisation rate, Edζ_ion,k∕dE, per logarithmic energy interval as a function of the energy at the column densities , 10²¹, 10²², and 10²³ cm⁻². Left plot: CR protons (models and ; cyan and blue lines, respectively). Right plot: CR primary electrons (red lines) and secondary electrons (models and ; cyan and blue lines, respectively). Solid circles and diamonds respectively denote the energies of primary CRs and secondary electrons that contribute most to the CR ionisation rate.