Cross-scale observations of expanding bifurcated current sheets in fast magnetic reconnection

Magnetic reconnection is a universal process that rapidly releases magnetic energy in plasmas, driving explosive phenomena in both space and laboratory environments. Yet a complete understanding of how reconnection proceeds in large-scale plasmas remains elusive. Here we show that expanding bifurcated current sheets (EBCS) are a recurring structure of quasi-steady reconnection. EBCS are double-peaked current layers that expand along the boundaries of the reconnection outflow, accompanied by localized electric fields and strong ion acceleration. We identify this structure consistently in laboratory experiments, spacecraft observations near Earth and the Sun, and large-scale kinetic simulations. Our results indicate that EBCS arise from two-fluid plasma dynamics and occur during quasi-steady antiparallel reconnection, particularly for plasmas with moderate plasma beta (0.1 < β ≲ 1, where β is the ratio of plasma thermal pressure to magnetic pressure). These findings link laboratory and space observations to the classical Petschek model and show how two-fluid dynamics can produce open reconnection outflows across widely different system sizes. Magnetic reconnection releases stored magnetic energy in space and laboratory plasmas. Here, the authors show that expanding bifurcated current sheets form during quasi-steady reconnection and help organize plasma outflows across scales.