The Interplay of Magnetically Dominated Turbulence and Magnetic Reconnection in Producing Nonthermal Particles
The Interplay of Magnetically Dominated Turbulence and Magnetic Reconnection in Producing Nonthermal Particles
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DOI：
10.3847/1538-4357/ab4c33
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发表时间：
2019-09
期刊：
影响因子：
--
通讯作者：
L. Comisso;L. Sironi
中科院分区：
文献类型：
--
作者：
L. Comisso;L. Sironi
磁化湍流和磁重联经常被用来解释从各种天体物理源观测到的非热辐射。通过全动力学二维和三维粒子格子模拟，我们研究了在磁控(或相当于“相对论”)对等离子体中产生非热粒子时，湍流和重联之间的相互作用。湍流演化的一个普遍副产品是产生具有幂函数能量范围的非热粒子光谱。对于较大的磁化强度和较强的湍流涨落，幂函数斜率p更加坚硬，甚至可以和p≲2一样坚硬。在高能截止点，粒子的拉莫尔半径与最大湍流涡旋的大小L相当。等离子体介导的重联在湍流等离子体中自洽地发生，控制着粒子注入的物理过程。然后，粒子被湍流涨落的随机散射进一步加速。平行电场所做的功--自然会在重联层中出现--是初始能量增加的主要原因，并与系统的磁化强度σ成正比，而随后的能量增益主导着高能粒子的整体能量，由湍流涨落的垂直电场提供动力。两阶段加速过程在粒子的俯仰角分布上留下了印记：低能粒子与场对齐，而最高能粒子优先垂直于场运动。随机加速度的能量扩散系数为Dγ∼0.1σ(c/L)γ2，其中γ为粒子洛伦兹因子。这导致了快速加速时间标度Tacc∼(3/σ)L/c。我们的发现对于理解高能天体物理源中非热粒子的产生具有重要意义。
Magnetized turbulence and magnetic reconnection are often invoked to explain the nonthermal emission observed from a wide variety of astrophysical sources. By means of fully kinetic 2D and 3D particle-in-cell simulations, we investigate the interplay between turbulence and reconnection in generating nonthermal particles in magnetically dominated (or, equivalently, “relativistic”) pair plasmas. A generic by-product of the turbulence evolution is the generation of a nonthermal particle spectrum with a power-law energy range. The power-law slope p is harder for larger magnetizations and stronger turbulence fluctuations, and it can be as hard as p ≲ 2. The Larmor radius of particles at the high-energy cutoff is comparable to the size l of the largest turbulent eddies. Plasmoid-mediated reconnection, which self-consistently occurs in the turbulent plasma, controls the physics of particle injection. Then, particles are further accelerated by stochastic scattering off turbulent fluctuations. The work done by parallel electric fields—naturally expected in reconnection layers—is responsible for most of the initial energy increase and is proportional to the magnetization σ of the system, while the subsequent energy gain, which dominates the overall energization of high-energy particles, is powered by the perpendicular electric fields of turbulent fluctuations. The two-stage acceleration process leaves an imprint in the particle pitch-angle distribution: low-energy particles are aligned with the field, while the highest-energy particles move preferentially orthogonal to it. The energy diffusion coefficient of stochastic acceleration scales as Dγ ∼ 0.1σ(c/l)γ2, where γ is the particle Lorentz factor. This results in fast acceleration timescales tacc ∼ (3/σ)l/c. Our findings have important implications for understanding the generation of nonthermal particles in high-energy astrophysical sources.