Trajectory of an electron in the (x, t) and (x − x_sh, v) spaces in the time interval 275 <  t ω_c <  300 for a simulation (T) with v₀/v_A = 10. The background (left frame) shows the amplitude of the magnetic field |B(x, t)|. The test-electron thermal velocity that is far upstream is The relatively high velocity of the traced particle (v/v_A ∼ 200) at t ω_c = 275 indicates that it has already been energized in the precursor and possibly during encounters with the shock at earlier times.