Ph.D. student CHENG Guanchong, Researcher NI Lei, and collaborators from the Solar Activity and CME Theory Research Group at Yunnan Observatories of the Chinese Academy of Sciences, using a combination of multi-wavelength high-resolution observations and numerical simulations, have provided clear and comprehensive evidence for plasmoid-mediated magnetic reconnection in the partially ionized environment of the lower solar atmosphere. The related research results, titled "Evidence for Plasmoid-Mediated Magnetic Reconnection during a Small-Scale Flare in the Partially Ionized Low Solar Atmosphere," were recently published in The Astrophysical Journal Letters.
From the cool photosphere to the hot corona, the solar atmosphere exhibits numerous magnetic reconnection eruption phenomena of various scales. During these activities, magnetic energy is rapidly converted into plasma kinetic energy, thermal energy, and accelerates high-energy particles. The observed solar eruption activities correspond to very high magnetic Reynolds numbers (approximately or far greater than 10^6). At the macroscopic magnetohydrodynamic (MHD) scale, neither the classical steady-state Sweet-Parker model nor the Petschek model can satisfactorily and self-consistently explain the rapid magnetic energy release processes during these solar eruptions. Numerous numerical simulation results indicate that during magnetic reconnection in high magnetic Reynolds number, low-density plasma environments, current sheets are often fragmented into numerous smaller-scale current sheets due to the presence of plasmoid instability (nonlinear stage of the tearing mode). The existence of multiple reconnection X-points allows for fast magnetic reconnection rates at the MHD scale, even without including fine physical mechanisms within the ion inertial scale, thus explaining the rapid magnetic energy release processes of solar eruptions. The cascade process of plasmoid instability is a key bridge connecting large-scale eruptions to the ultimate small-scale dissipation.
Plasmoid-mediated magnetic reconnection has been detected in plasma laboratories and the magnetosphere, and a significant amount of high-resolution observational data has also identified plasmoid structures moving away from and toward the solar surface in large-scale solar activities, such as in CME-flare current sheets. Although numerical simulations of partially ionized magnetic reconnection suggest that plasmoid instability also exists in the low-temperature magnetic reconnection processes of the lower solar atmosphere, observational evidence for plasmoid instability in reconnection under partially ionized conditions has been scarce due to observational limitations.
This study utilized high-resolution Hα band observations from the Goode Solar Telescope (GST) at Big Bear Solar Observatory, combined with SDO observations, magnetic field extrapolation, and high-precision two-dimensional numerical simulations, to track and diagnose a small-scale magnetic reconnection event spanning the lower solar atmosphere and the low corona. In a small-scale current sheet approximately 2 Mm long, this study discovered a plasmoid structure about 150 km in size in the Hα line wing imaging observations. This structure moved downward with a maximum speed of 24 km/s, then interacted with the post-reconnection loop region, reducing its speed and leading to brightening in multiple bands, forming a flare. Magnetohydrodynamic numerical simulations further confirmed this physical scenario.
The research results provide clear and comprehensive observational evidence for the existence of plasmoid-mediated magnetic reconnection in weakly ionized environments, advancing the study of magnetic reconnection mechanisms under partially ionized conditions. Additionally, this research fills a critical gap in the universal model of plasmoid instability being widespread in solar eruption activities, connecting various scales of solar flares from the lower chromosphere to the corona.
Contact:
CHENG Guanchong
Yunnan Observatories, CAS
e-mail:chengguanchong@ynao.ac.cn
