The journal Astronomy & Astrophysics has recently published a study from the Solar Activity and Coronal Mass Ejection (CME) Theory Research Group at the Yunnan Observatories of the Chinese Academy of Sciences. Led by Ph.D. student CHENG Guanchong and researcher NI Lei, the research utilized high-precision radiation magnetohydrodynamic (MHD) numerical simulations to study the fine physical processes of magnetic reconnection between newly emerging magnetic fields and background magnetic fields in the solar lower atmosphere. This study has revealed a connection between Ellerman bombs (EBs) and ultraviolet bursts (UV bursts), uncovering the formation mechanism of multi-temperature small-scale thermal explosions.
EBs and UV bursts are small-scale solar eruptions observed in the solar lower atmosphere. EBs, characterized by enhanced Hα line wing radiation, form at temperatures of a few thousand Kelvin in the lower chromosphere or upper photosphere. UV bursts, identified by significant Si IV band radiation, occur at temperatures above 20,000 Kelvin and are associated with intense solar activity. It is estimated that about 20% of UV bursts are linked to EBs, both arising from the same magnetic reconnection processes. However, the exact relationship between the two phenomena has remained unclear.
Building on earlier work of researcher NI Lei and collaborators, this study developed more realistic modules for radiative cooling and time-evolving ionization in MHD codes. These improvements allowed for a more accurate simulation of the conversion of magnetic energy to thermal energy and the temporal and spatial evolution of temperature. The study also employed the radiation transfer code RH1.5 to synthesize optically thick Hα and Si IV spectral profiles for comparison with observations.
In the simulation, emerging magnetic loops gradually brought high-density plasma into higher atmospheric layers. Initially, the current sheet between the magnetic loop and the background magnetic field had a temperature of only a few thousand Kelvin. When the line of sight crossed the current sheet, the synthesized Hα spectral profile exhibited typical Ellerman bomb characteristics, without significant Si IV radiation enhancement. In the subsequent stage, the current sheet emerged into the lower chromosphere, with some high-density plasma returning to the solar surface. The current sheet temperature increased, and turbulent magnetic reconnection caused by plasma instability led to highly uneven temperature and density distribution within the current sheet. The highest temperatures exceeded 100,000 Kelvin, while high-density, low-temperature regions remained only a few thousand Kelvin. High-temperature and low-temperature plasmas alternately mixed in the reconnection region, even within the same magnetic island. Low-temperature regions could appear above high-temperature regions. The synthesized Hα and Si IV spectral profiles crossing the current sheet showed typical characteristics of Ellerman bombs and ultraviolet bursts, with Si IV spectral profile widths exceeding 100 km/s.
These simulation results have indicated that plasma instability can occur in both EBs and UV bursts. If the reconnection magnetic field is strong enough, the resulting turbulent magnetic reconnection will cause these small-scale activities to alternately mix and appear in the same magnetic reconnection process. This research provides a new model for the formation of multi-temperature small-scale thermal explosions associated with EBs and UV bursts.
Contact:
CHENG Guanchong
Yunnan Observatories, CAS
e-mail:chengguanchong@ynao.ac.cn
