Stability transitions and dynamics of mesa patterns near the shadow limit ofreaction-diffusion systems in one space dimension

We consider a class of one-dimensional reaction-diffusion systems,\[\{{array}[uₜ=ε²uₓₓ+f(u,w)\\τ wₜ=Dwₓₓ+g(u,w){array}.\]with homogeneous Neumann boundary conditions on a one dimensional interval.Under some generic conditions on the nonlinearities $f,g$ and in the singularlimit ε→0, such a system admits a steady state for whichu consists of sharp back-to-back interfaces. For a sufficiently large Dand for sufficiently small τ, such a steady state is known to be stablein time. On the other hand, it is also known that in the so-called shadowlimit D→∞, patterns having more than one interface areunstable. In this paper we analyse in detail the transition between the stablepatterns when $D=O(1)$ and the shadow system when D→∞. Weshow that this transition occurs when D is exponentially large in ε and we derive instability thresholds D₁ D₂₃… such that a periodic pattern with $2K$ interfaces is stable ifD DK. We also study the dynamics of theinterfaces when D is exponentially large; this allows us to describe indetail the mechanism leading to the instability. Direct numerical computationsof stability and dynamics are performed, and these results are in excellentagreement with corresponding results as predicted by the asymptotic theory.