Researchers at the Yunnan Astronomical Observatory of the Chinese Academy of Sciences have recently revealed the mechanism behind the formation of small-scale thermal explosions in the lower atmosphere of the Sun. Their findings have been published in the international journal "Astronomy & Astrophysics."
Ellerman bombs and ultraviolet bursts are two small-scale magnetic reconnection activities commonly found in the lower atmosphere of the Sun, representing the smallest observable solar eruptive events. While both have similar sizes and lifetimes, there is a difference of 1 to 2 orders of magnitude in the increase in plasma temperature and released energy during their formation, with approximately 20% of ultraviolet bursts being associated with Ellerman bombs.
Previously, based on numerical simulation results, foreign researchers proposed that the upper and lower ends of a vertical current sheet correspond to the high-temperature ultraviolet bursts and low-temperature Ellerman bombs, respectively. These two small-scale activities can form within the same magnetic reconnection process but are always located in the mid-to-high chromosphere and photosphere. However, a series of earlier studies by researcher Ni Lei at the Yunnan Astronomical Observatory indicated that as long as the reconnected magnetic field exceeds 500 gauss, ultraviolet bursts can also occur in the lower chromosphere. Turbulent magnetic reconnection leads to an uneven distribution of temperature and density in the reconnection region, allowing high-temperature ultraviolet bursts and low-temperature Ellerman bombs to potentially coexist in the lower chromosphere.
Research Diagram. Provided by Yunnan Observatory
Building upon existing work, Dr. Cheng Guanchong and Researcher Ni Lei, from the Yunnan Observatory's research group on solar activity and coronal mass ejections, have developed a module that tracks the evolution of radiation cooling and ionization over time. This module enhances the realism of processes such as the conversion of magnetic energy into thermal energy and the evolution of temperature in space and time. They have also utilized high-precision radiation magnetohydrodynamic numerical simulations to study the intricate physical processes of magnetic reconnection between new emerging magnetic fields and background magnetic fields in the lower solar atmosphere.
In contrast to models proposed by foreign counterparts, the simulation results of this study suggest that plasma instabilities can occur in two types of small-scale activities in the lower solar atmosphere. If the reconnected magnetic field is strong enough, the resulting turbulent magnetic reconnection will cause these two small-scale activities to spatially mix and alternate within the same magnetic reconnection process. This provides a new model for the formation of multi-temperature small-scale thermal eruptions related to Ellerman bombs in ultraviolet bursts.
