Snapshots of the density (green), temperature (red), and gas velocity (grey) profiles of the first QPO cycle with the Chr–Λ setup; the accreted gas falls from the right to the left. The first line (between 1 and 353 s) corresponds to the first cycle. The second and third lines correspond to the beginning of the second cycle. Snapshots att = 71 s and 415 s arevery close: from this time, the cycle behaves like the previous one. A typical sequence is: growth of a hot slab of shocked material (t = 21 s), quasi-isochoric cooling at the slab basis (thermal instability, t = 168 s), start of the collapse of the post-shock structure (t = 336 s), impact of the collapsing material on the chromosphere (t = 354 s), launch of a new shock before the end of the collapse (t = 358 s), passing of the two shocks (t = 380 s), end of the collapse of the old structure (t = 386 s) and growth of the new slab (t = 415 s).