Total CR ionisation rate as a function of the H₂ column density. Theoretical models ℒ (lower solid black line), ℋ (dotted black line), and 𝓤 (upper solid black line). Expected values from models also include the ionisation due to primary CR electrons and secondary electrons. Observational estimates in diffuse clouds: solid down-pointing triangle (Shaw et al. 2008), solid up-pointing triangle (Neufeld et al. 2010), solid left-pointing triangles (Indriolo & McCall 2012), solid right-pointing triangles (Neufeld & Wolfire 2017), empty right-pointing triangles (Luo et al. 2023a), empty left-pointing triangles (Luo et al. 2023b); in low-mass dense cores: solid circles (Caselli et al. 1998), empty hexagons (Bialy et al. 2022), empty circle (Maret & Bergin 2007), empty pentagon (Fuente et al. 2016), empty up-pointing triangle (Redaelli et al. 2021); in high-mass star-forming regions: stars (Sabatini et al. 2020), black and grey small symbols (Sabatini et al. 2023), solid diamonds (de Boisanger et al. 1996), empty diamonds (van der Tak et al. 2000), empty thin diamonds (Hezareh et al. 2008), solid thin diamonds (Morales Ortiz et al. 2014); in circumstellar discs: empty squares (Cec-carelli et al. 2004); in massive hot cores: solid squares (Barger & Garrod 2020).