A research team from the Yunnan Observatories of the Chinese Academy of Sciences, has shed new light on the magnetic reconnection process driven by rapidly expanding plasma, using magnetohydrodynamic (MHD) numerical simulations. Their findings, published recently in Science China Physics, Mechanics & Astronomy, reveal previously unobserved fine structures and physical mechanisms underlying this fundamental phenomenon.

Magnetic reconnection—a process where magnetic field lines break and rejoin, releasing massive energy—is critical to understanding explosive events in plasmas, from laboratory experiments to solar flares and space weather. The team focused on how this process unfolds under rapid driving conditions, examining three distinct reconnection modes: flux pile-up, Sonnerup, and hybrid. These modes, they found, arise from variations in gas pressure and magnetic field strength within the inflow region, where plasma is drawn into the reconnection site.

Key to their discoveries was the behavior of the Spitzer diffusion region—the core area where reconnection initiates. Contrary to the simplified "X-point" model often assumed, simulations showed this region forms as a thin, elongated current sheet. At its ends, two pairs of slow-mode shocks (SS) emerge; these shocks create four distinct boundaries that separate inflowing plasma from the outflow, where energy is expelled.

Further analysis revealed more complex structures farther from the Spitzer region: two sets of rotational discontinuities (RDs) lie just inward of the slow shocks, forming a combined SS/RD structure. These RDs trigger a reversal in magnetic field direction within the outflow, producing a distinctive "W-shaped" magnetic field configuration. Notably, the team clarified that this field rotation is not caused by intermediate waves—a long-debated possibility—and confirmed that slow shocks are positioned outside the rotational discontinuities.

This study offers a novel perspective on rapidly driven magnetic reconnection through numerical experiments. It deepens our grasp of this process and carries significant theoretical weight for explaining energy-release phenomena in laboratory, solar, and space plasmas, the researchers noted.

The work was supported by funding from China's National Key R&D Program and the National Natural Science Foundation of China (Key Program), among other sources.