Three-dimensional Magnetic Reconnection Mediated with Plasmoids and the Resulting Multithermal Emissions in the Cool Atmosphere of the Sun

Context. Flux emergence is ubiquitous in the Sun's lower atmosphere. The emerging flux can reconnect with the pre-existing magnetic field. Aims. We aim to investigate plasmoid formation and the resulting multithermal emissions during the three-dimensional reconnection process in the lower solar atmosphere. Methods. We conducted 3D radiation magnetohydrodynamic (RMHD) simulations using the MURaM code, which incorporates solar convection and radiation. We simulated the emergence of a flat magnetic flux sheet that was introduced into the convection zone. For comparison with results previously reported from observations, we employed the RH1.5D code to synthesize H alpha and Si IV spectral line profiles and we synthesized the ultraviolet images using the optical thin methods. Results. Flux emergence took place as part of the imposed flux tube crossed the photosphere. In the lower solar atmosphere, magnetic reconnection occurred and formed thin, elongated current sheets. Plasmoid-like features appear as part of the reconnection process; this results in many small twisted magnetic flux ropes, which are expelled toward the two ends of the reconnection region. Consequently, hot plasma with a temperature exceeding 20 000 K and much cooler plasmas with a temperature below 10 000 K can coexist in the reconnection region. Synthesized images and spectral line profiles through the reconnection region display typical characteristics of reconnection occuring in the lower solar atmosphere, such as Ellerman bombs (EBs) and UV bursts. The cooler plasmas that show characteristics of EBs can be found above hot plasma and reach altitudes more than 2 Mm above the solar surface. Meanwhile, some hot plasma that features characteristics of UV bursts can extend downward to the lower chromosphere, approximately 0.7 Mm above the solar surface. Conclusions. Our simulation results indicate that the turbulent reconnection mediated with plasmoid instability can occur in small-scale reconnection events such as EBs and UV bursts. The coexistence of hot and much cooler plasmas in such a turbulent reconnection process can well explain the temporal and spatical connection of UV bursts with EBs.