Multi-Temperature Thermodynamics: A Multiplicity of Saha Equations
The multitemperature equilibrium model must be viewed in the framework of irreversible thermodynamics (Prigogine 1955). It applies to systems which can be divided in subsystems with weak interaction between each other, but with a rapid internal relaxation. In this case, each subsystem reaches an internal equilibrium, while the overall system is outside equilibrium, relaxing very slowly. The theory was originally developed for systems whose volume was geometrically divided in different parts, with high ratio volumes/surfaces, limiting exchange of mass and energy through their interfaces.