| 其他摘要 | The solar lower atmosphere is the source and key region for coronal heating, solar wind formation, and large-scale eruptions. It features complex magnetic field structures and highly dynamic plasma environments, constantly exhibiting various types of small-scale transient phenomena. These phenomena are crucial for understanding the physical properties of the chromosphere, the dynamic magnetic structures in the atmosphere, the anomalous heating of the solar atmosphere, and other large-scale activity phenomena. Ellerman bombs (EBs) and ultraviolet bursts (UV bursts) are two typical small-scale brightening phenomena in the solar lower atmosphere. They are generally believed to be primarily driven by magnetic reconnection triggered by emerging magnetic flux. Multi-wavelength coordinated observations indicate that some small-scale brightenings exhibit significant responses in both the H$\alpha$ line wing and the ultraviolet Si IV band. This study mainly focuses on the fine plasma physical processes and magnetic field structures during the formation of EBs and UV bursts, and the connections between these two small-scale transient phenomena.We enhanced the open-source NIRVANA code by incorporating an approximate model for handling radiative transfer processes in the photosphere and chromosphere, along with a time-dependent ionization module. In the photosphere, we adopted a simplified local thermodynamic equilibrium (LTE) model and the Saha equation. For the chromosphere, approximate radiation and ionization tables for crucial spectral lines such as H I, Ca II, and Mg II were utilized. The improved code retains NIRVANA's adaptive mesh capability while comprehensively considering the radiative processes in the solar lower atmosphere. However, the required computation time has increased by a factor of 3 to 10 under the same computational scale.Using the enhanced NIRVANA code, we simulated the physical process of magnetic reconnection between emerging magnetic fields and background magnetic fields in a strong magnetic field environment in two-dimensional space, based on the real gravity-stratified solar lower atmosphere. The simulation shows that the emerging magnetic field drives high-density plasma from the photosphere into the magnetic reconnection region. Initially, the temperature of the reconnecting current sheet is below 8000 K. As the current sheet becomes more vertical, high-density plasma gradually descends. Plasma instability in the reconnection region leads to an uneven density distribution, with some low-density plasma being heated to over 20,000 K, reaching temperatures as high as 100,000 K. In the turbulent reconnection region, plasma at different temperatures alternately appears, even within the same magnetic island, with high-temperature regions appearing below low-temperature regions. Crossing the magnetic reconnection region, the synthesized Si IV emission intensity can exceed 10$^6$ erg s$^{-1}$ sr$^{-1}$ cm$^{-2}$ Å$^{-1}$, and the line profile width can exceed 100 km$\cdot s^{-1}$. The H$\alpha$ line profile exhibits typical Ellerman bomb characteristics. The turbulent current sheet always exists in a dense plasma environment with an optical depth greater than 6.5$\times$10$^{-5}$. The results suggest that Ellerman bombs and ultraviolet bursts may form in the same turbulent magnetic reconnection process, providing a reasonable explanation for the related ultraviolet bursts.Utilizing the MURaM code in a more realistic convective-driven solar atmosphere and background magnetic field environment, we first revealed the three-dimensional fine structure of plasma instability-mediated magnetic reconnection in the solar lower atmosphere, where high-temperature material exceeding 20,000 K and low-temperature plasma of a few thousand K alternately appear in space. Enhanced radiation was observed in the synthesized H$\alpha$ and Si IV bands, consistent with the observational features of ultraviolet bursts associated with Ellerman bombs. Three-dimensional simulations further confirm the results of two-dimensional simulations. Additionally, small-scale magnetic flux ropes formed by magnetic reconnection exhibit richer characteristics and may play a crucial role in transporting magnetic rope structures to higher atmospheric layers.Through observations using the Goode Solar Telescope (GST) in the H$\alpha$ band, we discovered a plasmoid structure about 150 km in size within a small-scale current sheet approximately 2 Mm long. This structure moved downward with a maximum speed of km$\cdot s^{-1}$, then interacted with the post-reconnection loop region, reducing its speed and leading to brightening in multiple bands, forming a flare. Nonlinear force-free field (NLFFF) extrapolation results indicate that the active region exhibits distinct magnetic field topology characteristic of magnetic reconnection. These observational evidences support the view that plasmoid-mediated magnetic reconnection exists in the solar lower atmosphere.In summary, we conducted high-resolution numerical simulations using the upgraded NIRVANA magnetohydrodynamic code to study the fine physical processes and radiative characteristics of magnetic reconnection between emerging magnetic fields and background magnetic fields. We proposed that turbulent magnetic reconnection caused by plasmoid instability leads to the alternating mixing of Ellerman bombs and ultraviolet bursts, two different temperature sub-arcsecond small-scale activities, within the same magnetic reconnection process in the lower atmosphere. Utilizing the world-class radiation magnetohydrodynamic code MURaM, we carried out the highest resolution three-dimensional simulations to date for this type of small-scale activity, further validating the results of the two-dimensional simulations and the proposed model. Finally, high-resolution H$\alpha$ band observations provided clear and comprehensive evidence for plasmoid-mediated magnetic reconnection in the lower solar atmosphere. Through this research, we developed modules for radiation magnetohydrodynamic codes suitable for the solar lower atmosphere, advancing the study of fine physical processes and mechanisms of magnetic reconnection in low-temperature, partially ionized environments. This work provides new insights into the formation mechanisms of sub-arcsecond multi-wavelength brightening phenomena in the solar lower atmosphere. |
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