Researchers including Dr. Abdullah Zafar, Prof. NI Lei and Prof. LIN Jun from the Yunnan Observatories of the Chinese Academy of Sciences discovered the faster dynamic Petschek-like magnetic reconnection in the partially ionized lower solar atmosphere, based on magnetohydrodynamic (MHD) simulations. The study was published in The Astrophysical Journal.

Magnetic reconnection refers to the process in a magnetized plasma where the magnetic field topology changes. It can explain various phenomena of magnetic energy release throughout the universe. In the partially ionized lower atmosphere of the Sun (photosphere and chromosphere), magnetic reconnection triggers many transient events which are major sources of solar atmospheric heating.

The Sweet-Parker model fails to explain energy release in solar flares, whereas the Petschek model achieves the fast reconnection rates but is unstable for uniform resistivity. Both steady-state models assume stable current sheets, but MHD simulations suggest that plasmoid instabilities in high Lundquist-number systems lead to fragmented current sheets and a reconnection rate on the order of 0.01, which is still an order of magnitude lower than the maximum observed rates.

In this study, researchers employed single-fluid radiative MHD simulations to study the dynamic reconnection processes at various altitudes in the partially ionized lower solar atmosphere, from the photosphere to the base of the corona.

The simulation results showed a sudden drop in plasma temperature and density at the main X-point during the reconnection process, which was caused by intense radiative cooling, the ejection of hot plasma near the X-point, or the motion of the X-point from a hot, dense area to a cooler, less dense area.

This significant decrease in temperature and density led to a reduction in pressure and an increase in magnetic diffusivity at the X-point, which consequently triggered dynamic Petschek-like reconnection that occurred after plasmoids were wiped out from the reconnection current sheet.

This study was the first to demonstrate the physical process in a partially ionized environment where reconnection transitioned from plasmoid-mediated to explosive faster dynamic Petschek-like reconnection. The maximum reconnection rates reached above 0.06, approximately three times larger than the plasmoid-dominated rate. The dynamic Petschek-like reconnection and the reconnection dominated by plasmoid instabilities alternated throughout the reconnection process.

This study revealed the physical mechanisms of eruptive fast magnetic reconnection in partially ionized solar plasma, and achieved a reconnection rate close to the fastest observed values. It will also help people to understand the mechanisms of rapid magnetic energy release in other partially ionized environments such as interstellar media, protoplanetary disks, and laboratories.