分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Magnetic reconnection drives explosive particle acceleration in a wide range of space and astrophysical applications. The energized particles often include multiple species (electrons, protons, heavy ions), but the underlying acceleration mechanism is poorly understood. In-situ observations of these minority heavy ions offer a more stringent test of acceleration mechanisms, but the multi-scale nature of reconnection hinders studies on heavy-ion acceleration. Here we employ hybrid simulations (fluid electron, kinetic ions) to capture 3D reconnection over an unprecedented range of scales. For the first time, our simulations demonstrate nonthermal acceleration of all available ion species into power-law spectra. The reconnection layers consist of fragmented kinking flux ropes as part of the reconnection-driven turbulence, which produces field-line chaos critical for accelerating all species. The upstream ion velocities influence the first Fermi reflection for injection. Then lower charge/mass species initiate Fermi acceleration at later times as they interact with growing flux ropes. The resulting spectra have similar power-law indices $(p\sim4.5)$, but different maximum energy/nucleon $\propto($charge/mass$)^\alpha$, with $\alpha\sim0.6$ for low plasma $\beta$, and with $p$ and $\alpha$ increasing as $\beta$ approaches unity. These findings are consistent with observations at heliospheric current sheets and the magnetotail, and provide strong evidence suggesting Fermi acceleration as the dominant ion-acceleration mechanism.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Astrophysical plasmas have the remarkable ability to preserve magnetic topology, which inevitably gives rise to the accumulation of magnetic energy within stressed regions including current sheets. This stored energy is often released explosively through the process of magnetic reconnection, which produces a reconfiguration of the magnetic field, along with high-speed flows, thermal heating, and nonthermal particle acceleration. Either collisional or kinetic dissipation mechanisms are required to overcome the topological constraints, both of which have been predicted by theory and validated with in situ spacecraft observations or laboratory experiments. However, major challenges remain in understanding magnetic reconnection in large systems, such as the solar corona, where the collisionality is weak and the kinetic scales are vanishingly small in comparison to macroscopic scales. The plasmoid instability or formation of multiple plasmoids in long reconnecting current sheets is one possible multiscale solution for bridging this vast range of scales, and new laboratory experiments are poised to study these regimes. In conjunction with these efforts, we anticipate that the coming era of exascale computing, together with the next generation of observational capabilities, will enable new progress on a range of challenging problems, including the energy build-up and onset of reconnection, partially ionized regimes, the influence of magnetic turbulence, and particle acceleration.
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