Ionisation rate at the shock surface in a jet as a function of the molecular hydrogen column density for models and described in the main text in the case of a parallel shock. Solid and dashed lines show the values of ζ neglecting and accounting for gyromotion effects, respectively. The dotted line displays ζ for model when the upstream diffusion coefficient is 30 times higher than the Bohm coefficient, without including any further attenuation due to gyromotion. The dash-dotted line shows ζ for model with η reduced by a factor of about ten with respect to the value in Table 2.