Stable spiral structures and their interaction in two-dimensional excitable media.

We study properties and interactions of stable spiral structures in two-dimensional excitable media modeled by equations of FitzHugh-Nagumo type. The presented results rely on the media ability to support unexcited spiral cores. Comparative frequency and dynamics analysis is done for a single spiral, an oppositely charged bound pair, and multiarmed spirals. Higher-frequency structures can attract, repel, or break up structures with lower frequencies. All pairwise reactions are described. A three-armed spiral and a two-armed spiral of opposite charge can form a stable "molecule." A system with high initial vortex concentration evolves according to the described reactions and exhibits three levels of self-organization. In our simulations such evolution led to formation and domination of persistent three-armed spirals.