Close-up views of some quantities of the reference model around the cold thread and the PCTR: a) Mass fractions of protons (ionized hydrogen), singly ionized helium, and doubly ionized helium. b) Ambipolar diffusion coefficient. Only half the domain with s ≥ 0 is displayed due to the symmetry of the profiles. The discontinuous derivatives of the ambipolar diffusion coefficient are caused by the first-order interpolation used to implement the hydrogen ionization degree from the tabulated values in Heinzel et al. (2015).