Φ-averaged profiles of the radially averaged Lorentz factor (top) and isotropic equivalent energy (bottom) of the jet head as functions of Θ at time t₁, that is shortly after the beginning of our Cartesian simulation. The three profiles in each panel refer to the results obtained for different threshold fractions of the maximum dE/dt adopted to define the radial extension of the jet head (see Sect. 2.3), namely 10%, 15%, and 20%.