Evolution of flare ribbons depicted from the (a, c) temperature and (b, d) QSL distributions. Temporal evolution of the (a) temperature and (b) squashing degree, Q, of QSLs on the bottom plane. Panels (c) and (d) show the time–distance diagram of temperature and Q along the blue line in Panel (a3), respectively. Two wave-like fronts in Panels (a3) and (a4) correspond to the shock in Figure 4c. The artifacts of Q near the side boundaries arise from the finite size of the computational domain. However, they do not give rise to significant heating fronts, electric currents, or Lorentz forces, and thus they have only a small impact on the derived conclusions. The box size is equal to that in plots (e) and (f) of Figure 3.