Pulse bifurcations and instabilities in an excitable medium: Computations in finite ring domains - art. no. 046212

We investigate the instabilities and bifurcations of traveling pulses in a model excitable medium; in particular, we discuss three different scenarios involving either the loss of stability or disappearance of stable pulses. In numerical simulations beyond the instabilities we observe replication of pulses ("backfiring") resulting in complex periodic or spatiotemporally ch aotic dynamics as well as modulated traveling pulses. We approximate the li near stability of traveling pulses through computations in a finite albeit large domain with periodic boundary conditions. The critical eigenmodes at the onset of the instabilities are related to the resulting spatiotemporal dynamics and "act" upon the back of the pulses. The first scenario has been analyzed earlier [M.G. Zimmermann et al., Physica D 110, 92 (1997)] for hi gh excitability (low excitation threshold): it involves the collision of a stable pulse branch with an unstable pulse branch in a so-called T point. I n the framework of traveling wave ordinary differential equations, pulses c orrespond to homoclinic orbits and the T point to a double heteroclinic loo p. We investigate this transition for a pulse in a domain with finite lengt h and periodic boundary conditions. Numerical evidence of the proximity of the infinite-domain T point in this setup appears in the form of two saddle node bifurcations. Alternatively, for intermediate excitation threshold, a n entire cascade of saddle nodes causing a "spiraling" of the pulse branch appears near the parameter values corresponding to the infinite-domain T po int. Backfiring appears at the first saddle-node bifurcation, which limits the existence region of stable pulses. The third case found in the model fo r large excitation threshold is an oscillatory instability giving rise to " breathing," traveling pulses that periodically vary in width and speed.