Fig. 8.

The slow flow on S in the vicinity of the two folds $F_1$ and $F_2$. The folds $F_1$ and $F_2$ mark the transition of the orbit segment between the two saddle-unstable sheets $S_1^r$ and $S_2^r$; the direction of flow on S is indicated by the arrows where hotter colours correspond to vectors with larger magnitudes. The attracting fold $F_1$ is dark green, and the repelling fold $F_2$ is light green. Panels (a), (b) and (c) show the same perspective with orbit segments that almost reach $F_1$, reach $F_1$ via the attracting sheet $S_2^a$ and reach $F_2$ with a jump from $S_1^r$ to $S_2^r$, respectively.