Efficient Nonthermal Ion and Electron Acceleration in 3D Magnetic Reconnection

Solar flare and Earth’s magnetotail observations show simultaneous acceleration of ions and electrons into power-law energy distributions extending to high energy. This suggests a common magnetic-reconnection acceleration process but the underlying physics is not well understood. During magnetic reconnection, energetic particles undergo a universal Fermi acceleration process involving the curvature drift of particles. However, the efficiency of this mechanism is limited by the trapping of energetic particles within flux ropes. Using 3D fully kinetic simulations, we demonstrate that the flux-rope kink instability leads to field-line chaos in weak-guide-field regimes where the Fermi mechanism is most efficient, thus allowing particles to transport out of flux ropes and undergo further acceleration. As a consequence, both ions and electrons form clear power laws which contain a significant fraction of the released energy. The low-energy bounds, which control the nonthermal energy contents, are determined by the injection physics, while the high-energy cutoffs are limited only by the system size. In contrast, in the higher-guide-field regimes, field-line chaos and efficient acceleration comes from 3D overlapping oblique tearing flux ropes of large sizes. As a result, both species also form power laws consistent with Fermi acceleration, with indices softer than weak-guide-field regimes. Interestingly, these oblique flux ropes are also subject to kink instability. These basic results have strong relevance to observations of nonthermal particle acceleration in both the solar corona and magnetotail.

This work is in collaboration with Fan Guo, Bill Daughton, Xiaocan Li, Hui Li.