Case of a parallel shock in jets: ionisation fraction, x, versus total hydrogen density, n_H, for different combinations of initial parameters. The magnetic field strength varies from 50 μG to 1 mG (from left to right), while the shock velocity varies from 40 to 160 km s^-1 (from top to bottom). The temperature is assumed equal to 10⁴ K and the shock distance from the protostar and its transverse radius are R_sh = 100 AU and R_⊥ = 10 AU, respectively. The colour map shows the values of E_max in the case of a parallel shock when the conditions imposed by Eqs. (4) and (18) are simultaneously satisfied. Cyan contours delimit the regions where E_max reaches its maximum asymptotic value shown in GeV in each subplot. Vertically hatched regions refer to combinations of parameters corresponding to strong wave damping (R< 1). The two solid white stars in the upper left and lower right plots show the values of n_H and x considered for the evaluation of the emerging spectra (models and in Fig. 4).