Fast Particle Acceleration in Three-dimensional Relativistic Reconnection
Fast Particle Acceleration in Three-dimensional Relativistic Reconnection
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DOI：
10.3847/1538-4357/ac2e08
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发表时间：
2021-04
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影响因子：
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通讯作者：
Hao Zhang;L. Sironi;D. Giannios
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作者：
Hao Zhang;L. Sironi;D. Giannios
磁重联是产生高能粒子的主要机制之一。我们使用大规模的3D粒子格内模拟来研究磁控(σ=10)对等离子体中重联的高能粒子的能化物理。我们确定了一种只能在3D模式下运行的加速机制。对于弱引导场，三维等离子体/通量绳沿着电流的z方向延伸，其长度与它们的横截面半径相当。与2D模拟中的粒子被掩埋在等离子体中不同，在3D中我们发现，具有γ≳3σ的一小部分粒子可以通过沿z移动从等离子体中逃逸，因此它们可以体验上游区域的大尺度场。这些“自由”粒子优先沿采样层两侧的类似斯皮塞的轨道沿z移动，并在时间上线性加速-它们的洛伦兹因子比例为γ∝t，与2D中的γ∝t形成对比。能量增益率接近∼EE rec，其中E rec≃0.1B 0是重联电场，B0是上游磁场。自由粒子的光谱是硬的，dNFree/dγ∝γ−1.5，包含∼20%的耗散磁能，与磁畴大小无关，并扩展到截止能量随盒大小线性缩放。我们的结果表明，GRB和AGN喷注中的相对论重联可能是产生超高能宇宙线的一种很有前途的机制。
Magnetic reconnection is invoked as one of the primary mechanisms to produce energetic particles. We employ large-scale 3D particle-in-cell simulations of reconnection in magnetically dominated (σ = 10) pair plasmas to study the energization physics of high-energy particles. We identify an acceleration mechanism that only operates in 3D. For weak guide fields, 3D plasmoids/flux ropes extend along the z-direction of the electric current for a length comparable to their cross-sectional radius. Unlike in 2D simulations, where particles are buried in plasmoids, in 3D we find that a fraction of particles with γ ≳ 3σ can escape from plasmoids by moving along z, and so they can experience the large-scale fields in the upstream region. These “free” particles preferentially move in z along Speiser-like orbits sampling both sides of the layer and are accelerated linearly in time—their Lorentz factor scales as γ ∝ t, in contrast to γ∝t in 2D. The energy gain rate approaches ∼eE rec c, where E rec ≃ 0.1B 0 is the reconnection electric field and B 0 the upstream magnetic field. The spectrum of free particles is hard, dNfree/dγ∝γ−1.5 , contains ∼20% of the dissipated magnetic energy independently of domain size, and extends up to a cutoff energy scaling linearly with box size. Our results demonstrate that relativistic reconnection in GRB and AGN jets may be a promising mechanism for generating ultra-high-energy cosmic rays.