A Magnetic Reconnection model for Hot Explosions in the Cool Atmosphere of the Sun

Abstract

Context. UV bursts and Ellerman bombs are transient brightenings observed in the low solar atmospheres of emerging flux regions. Magnetic reconnection is believed as the main mechanism to result in the formations of the two activities, which are usually formed far apart from each other. However, the observations also 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 partially ionized lower solar atmosphere. Aims. We have studied magnetic reconnection between the emerging magnetic flux and back ground magnetic fields in the partially ionized and highly stratificated low solar atmosphere. We aim to explain the multi-thermal characteristics of UV bursts and find out whether EBs and UV burst can be generated in the same reconnection process, and how they are related with each other. We also aim to unearth the important small-scale physics in these events. Methods. The single-fluid MHD code NIRVANA was used to perform simulations. The background magnetic fields and emerging fields at the solar surface are reasonably strong. The initial plasma parameters are based on the C7 atmosphere model (Avrett & Loeser 2008). Cases with different resolutions have been simulated, and the effects of ambipolar diffusion, radiative cooling and heat conduction have been included. The current density, plasma density, temperature and velocity distributions in the main current sheet region have been analyzed. The Si iv emission spectrum has been synthesized. Results. 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. (2019b), in which UV bursts have a tendency to locate at the higher heights of corresponding EBs and all the EBs have partial overlap with corresponding UV burst in space. 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...