Advances have been made in research on the emergence of magnetic flux leading to catastrophes in the structure of the coronal magnetic field

Recently, The Astrophysical Journal, an international journal of astronomy, published the latest research results of the “Solar Activity and CME Theory Research Group” of the Yunnan Astronomical Observatory of the Chinese Academy of Sciences, which was jointly completed by master’s student Chen Yuhao, Dr. Ye Jing, Associate Researcher Mei Zhixing and others. They studied the physical process of magnetic flux emergence causing the helioter structure to shift sideways, and their results showed that the new floating flux will trigger asymmetry in the coronal magnetic field, and under such a complex magnetic field structure, the dark strip structure in the corona will still undergo catastrophes.

Solar eruptions are the most intense energy releases in the solar system, with a typical burst of activity equivalent to billions of giant hydrogen bombs exploding simultaneously. Cataclysm theory is one of the mainstream theories explaining solar eruptions, describing the physical mechanism by which a steady-state dark strip structure is gradually out of balance driven by the evolution of the photosphere. In the past 20 years, Lin Jun and others at the Yunnan Observatory have successfully given a theoretical catastrophe model of a solar eruption driven by magnetic current emergence, but in order to obtain an analytical solution, the model has been oversimplified. In order to understand the complex solar eruption process, it is necessary to study whether the emergence of magnetic currents can lead to the catastrophe process of dark bars under more realistic physical conditions.

The researchers performed a two-dimensional numerical simulation of magnetohydrodynamics (Figure 1), in which the magnetic field structure was not symmetrical, so the magnetic rope could move in any direction. The results show that when the magnetic rope radius is relatively small, the magnetic rope can be quickly adjusted to a new equilibrium at the initial position, and can evolve quasi-statically with the emergence of the magnetic field, and when the magnetic rope evolves to a certain critical point, the catastrophe occurs. When the magnetic rope radius is relatively large, the magnetic rope cannot find any balance. When a catastrophe occurs, two current sheets with opposite current directions are formed in different forms (Figure 2): when the magnetic rope is unbalanced and moves upwards, the magnetic rope will stretch the magnetic field lines in the opposite direction around them to form the first current sheet; When the magnetic rope moves around, it squeezes the nearby magnetic field and forms a second current sheet. The magnetic reconnection within the two current sheets affects the force process of the magnetic rope, eventually causing a failed burst. These results suggest that catastrophes still occur when the magnetic field is asymmetrical and non-powerless field effects are taken into account, with the consequence of lateral or non-radial bursts of the magnetic rope.

Figure 1: Initial magnetic field structure diagram, including: dipole one and dipole two (the former produces a background magnetic field that does not change over time, the latter gradually increasing in intensity to simulate the emergence of magnetic flux), magnetic rope (used to simulate the structure of dark bars in the corona), and mirror current of the magnetic rope (related to boundary conditions)

Figure 2: The evolution of the current structure, CS1 and CS2 are two current sheets with opposite current directions and different formation methods

This work has been supported by the Strategic Pilot (A) Research Project of the Chinese Academy of Sciences, the Key Project of the National Natural Science Foundation of China, the Yunnan Provincial High-level Talent Training Support Program – Yunling Scholars Project, and the Yunnan Provincial Heliophysical Scientist Studio Project. Numerical simulation, algorithm development and image processing were all completed at the Computational Heliophysics Laboratory of Yunnan Astronomical Observatory. (Source: Yunnan Astronomical Observatory, Chinese Academy of Sciences)

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