Dynamics of tertiary plasmoids in asymmetric double current sheets

The dynamics of tertiary plasmoids in asymmetric double current sheets is investigated by means of resistive magnetohydrodynamic simulations. It is found in simulations that increasing global asymmetry significantly affects the transition from the slow to the fast reconnection regime and alters the kinetic-energy growth rate during the slow nonlinear phase. In contrast, the fast reconnection phase is mainly governed by local plasmoid interactions. The formation of a giant plasmoid dominates the first kinetic-energy peak, while further fragmentation of the tertiary current sheet and the resulting plasmoid chains determine the generation and evolution of tertiary magnetic islands. Four typical evolutionary patterns of plasmoid chains are identified, each corresponding to distinct tertiary-island morphologies and energy-release characteristics in the later stages. Stable and fully developed tertiary islands tend to form under moderate β and sufficiently strong positive asymmetry, where reduced magnetic tension and enhanced thermal pressure promote plasmoid coalescence and magnetic flux accumulation. Fully developed tertiary islands block the downstream propagation of the giant plasmoid, thus suppressing further reconnection. In addition, weakly developed tertiary structures allow continued reconnection and enhanced late-stage energy release. Based on our statistics and the corresponding energy differences, it is found that the full development of tertiary magnetic islands is closely related with the energy difference, providing a predictive measure for island growth.