Impact of helium on the temperature equilibration. The solid lines correspond to the case with no helium, while the dashed lines show the temperature evolution when the He abundance is set to 0.084 of hydrogen (Lodders et al. 2009). The main effect of helium is (i) the increase of the mean atomic weight of plasma and, as a consequence, lower downstream temperature of ions (visible for τ < 10¹² cm⁻³ s) and (ii) keeping Fe ions slightly hotter at τ > 10¹² cm⁻³ s, since the helium ions have the longest equilibration time.