(color online) The superposition of 1D diagrams of the wake field structure and the plasma density during the propagation of the positively chirped pulse in plasma at the times of: (a) t = 400 fs, (b) t = 510 fs, and (c) t = 570 fs. The blue curves show the electrostatic field of the wake field and the black curves specify the density distribution of the injected electrons. Also the green and red regions determine the acceleration phase and deceleration phase of the wake field, respectively. (d) The energy spectrum of the injected electrons at the corresponding times of: t = 410 fs, t = 510 fs, and t = 570 fs.