Progress has been made in the acceleration of charged particles in turbulent magnetic reconnection current sheets

Recently, The Astrophysical Journal, an international journal of astronomy, published the latest research results of the “Solar Activity and CME Theoretical Research Group” of Yunnan Astronomical Observatory, which was jointly completed by Li Yan and other collaborators in the group. They studied in detail the acceleration process of charged particles in turbulent magnetic reconnection current sheets, and gave some new results of particle acceleration.

Solar flares are the most intense energy release process in the Sun’s atmosphere (typical bursts can reach 1025 joules), which quickly convert magnetic energy into plasma heat, kinetic energy, and accelerate charged particles in a very short time. Observational studies have shown that high-energy non-thermal particles account for 10%-50% of the released energy. The induced electric field generated by the magnetic reconnection current sheet is an effective mechanism to accelerate high-energy particles, but the long current sheet is usually unstable and turbulent, which will produce various plasma instability, the most important manifestation of which is the formation and merger of magnetic islands. Studying the motion of charged particles in such complex electromagnetic field structures is necessary to understand the detailed process of particle acceleration.

In this study, a self-consistent electromagnetic field structure was obtained through numerical calculation of magnetohydrodynamic equations. Then, by experimenting with particle methods, the acceleration of particles in turbulent current sheets is studied. The results show that the energy spectra of electrons and protons show a single power-law spectrum. The energetic components of the power-law spectrum consist of particles trapped in a closed magnetic field, while escaping and partially captured particles contribute to the lower energy components of the energy spectrum. The spectral index changes soft-hard-soft with the evolution of magnetic reconnection (Figure 1). Due to the motion of the magnetic island, there is an opposite electric field at both ends of the magnetic island. As a result, curvature drift in this region has little effect on particle acceleration and energy increase, while gradient drift acceleration plays a very important role in particle acceleration (Figure 2). In addition, due to the existence of magnetic islands, the separation of protons and electrons cannot be seen as clearly as in the X-point type when considering the guide field, and only particles in the high-energy part can observe this phenomenon (Figure 3).

Figure 1 Evolution of proton (left) and electron (right) energy spectra

Figure 2 Left: Proton trajectory in the xy plane. Right: Changes in the direction of the kinetic energy and total kinetic energy (black) of the parallel (red) and perpendicular (blue) magnetic fields of the particles.

Figure 3 Distribution of high-energy protons (left) and electrons (right).

The research was supported by the National Natural Science Foundation of China, the Strategic Leading Science and Technology Project of the Chinese Academy of Sciences, and the Lin Jun Scientist Studio of Yunnan Province. Scientific calculations were completed at the Laboratory of Computational Heliophysics of Yunnan Astronomical Observatory. (Source: Yunnan Astronomical Observatory, Chinese Academy of Sciences)

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