Transition from Plasmoid Chains to Petschek Reconnection in Low-Magnetization Pair Plasmas: Effects on Particle Energization

Reconnecting plasmas can be divided into two basic geometries: single X-point configurations and multiple X-point plasmoid chains. We investigate the transition between these two regimes using 2D particle-in-cell simulations. Using unprecedentedly large pair simulations with outflow boundary conditions, we find that by lowering the magnetization σ from relativistic (σ » 1) to subrelativistic (σ < 1), the plasmoid chains observed in the former case are replaced by a single X-point Petschek geometry in the latter. In this configuration, laminar exhausts bounded by slow-mode shocks emanate from a single X-point. The reason for this change is the reduction in secondary tearing rate as the magnetization is reduced: below a critical threshold, there are too few plasmoids to disrupt the formation of wide-angle exhausts, which thereafter dominate the domain. This transition from multiple X-point to single X-point reconnection further has a decisive effect on particle energization: we show that while high-σ plasmoid chains produce robust power-law energy spectra, low-σ Petschek exhausts merely heat incoming plasma and yield negligible nonthermal acceleration. Our findings are important for determining the large-scale geometries and particle energy spectra for a wide range of reconnecting systems.