Left panel: average proton surface densities of sheaths σ_sh, Eq. (3), as a function of the average MC speeds (V_mc) of all events. The vertical dashed lines indicate the thresholds between the three groups shown in Figs. 4 and 6. The quantity σ_sh is a proxy of the accumulated material ahead of MCs, within the sheaths. On average, slower MCs drag more material. Right panel: histogram of σ_sh for the 44 events.