Detailed look at the onset of PMJs. Two identical space-time diagrams are shown, with the color-scaling (shown at left) relative to the maximum intensity of the PMJ at t = 0. In the left diagram, horizontal blue and black lines serve as contours at the 90% and 80% intensity levels, respectively, relative to the maximum of the PMJ. In the right space-time diagram, the vertical lines with arrowheads show the full width at 25% (FWQM, thick purple) and 50% (FWHM, thin yellow) of the maximum intensity at that time step. Right panel: intensity profiles for three selected time steps. Time is relative to the maximum intensity of the PMJ (t = 0 s). The PMJs are identified by the letters in the upper left corner, which refer to PMJ A and B in Fig. 2 and PMJ E and F in Fig. 4.