A magnetic reconnection model for hot explosions in the cool atmosphere of the Sun

UV bursts and Ellerman bombs are transient brightenings observed in the low solar atmospheres of emerging flux regions. Observations have discovered the cospatial and cotemporal EBs and UV bursts, and their formation mechanisms are still not clear. The multi-thermal components with a large temperature span in these events challenge our understanding of magnetic reconnection and heating mechanisms in the low solar atmosphere. We have studied magnetic reconnection between the emerging and background magnetic fields. The initial plasma parameters are based on the C7 atmosphere model. After the current sheet with dense photosphere plasma is emerged to 0.5 Mm above the solar surface, plasmoid instability appears. The plasmoids collide and coalesce with each other, which makes the plasmas with different densities and temperatures mixed up in the turbulent reconnection region. Therefore, the hot plasmas corresponding to the UV emissions and colder plasmas corresponding to the emissions from other wavelenghts can move together and occur at about the same height. In the meantime, the hot turbulent structures basically concentrate above 0.4 Mm, whereas the cool plasmas extend to much lower heights to the bottom of the current sheet. These phenomena are consistent with the observations of Chen et al. 2019, ApJL. The synthesized Si IV line profiles are similar to the observed one in UV bursts, the enhanced wing of the line profiles can extend to about 100 km s−1. The differences are significant among the numerical results with different resolutions, which indicate that the realistic magnetic diffusivity is crucial to reveal the fine structures and realistic plasmas heating in these reconnection events. Our results also show that the reconnection heating contributed by ambipolar diffusion in the low chromosphere around the temperature minimum region is not efficient.