Electron and Proton Acceleration in Trans-relativistic Magnetic Reconnection: Dependence on Plasma Beta and Magnetization
Electron and Proton Acceleration in Trans-relativistic Magnetic Reconnection: Dependence on Plasma Beta and Magnetization
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DOI：
10.3847/1538-4357/aac820
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发表时间：
2018-03
期刊：
影响因子：
--
通讯作者：
D. Ball;L. Sironi;F. Özel
中科院分区：
文献类型：
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作者：
D. Ball;L. Sironi;F. Özel
通过磁重联的非热电子加速被认为在驱动黑洞周围辐射低效吸积流的可变x射线发射中起重要作用。磁重联的反相对论状态——磁化强度σ，定义为磁能密度与焓密度之比，为~ 1——在这样的流动中经常遇到。通过大量的二维细胞内粒子模拟，我们研究了反相对论状态下的电子和质子加速。我们重点研究了电子能谱对σ和质子β（质子热压与磁压之比）的依赖关系。我们发现在重连接区域的电子能谱是非热的，可以用幂律来建模。在低β处，斜率p与β无关，并随着σ的增大而变硬，达到1.8 + 0.7 / σ。电子主要被x点的非理想电场加速，要么在初始电流层中，要么在合并磁岛之间产生的电流片中。在较高的β值下，电子幂律变陡，所有σ值下的电子能谱最终接近麦克斯韦分布。在β值接近βmax≈1/4σ时，当电子和质子在重联之前都处于相对热的状态时，两种物质的光谱在高能量下都显示出额外的成分，其中含有百分之几的粒子。这些粒子通过类似费米的过程在重连接流出和静止磁岛之间弹跳而加速
Nonthermal electron acceleration via magnetic reconnection is thought to play an important role in powering the variable X-ray emission from radiatively inefficient accretion flows around black holes. The trans-relativistic regime of magnetic reconnection—where the magnetization σ, defined as the ratio of magnetic energy density to enthalpy density, is ∼1—is frequently encountered in such flows. By means of a large suite of two-dimensional particle-in-cell simulations, we investigate electron and proton acceleration in the trans-relativistic regime. We focus on the dependence of the electron energy spectrum on σ and the proton β (the ratio of proton thermal pressure to magnetic pressure). We find that the electron spectrum in the reconnection region is nonthermal and can be modeled as a power law. At low β, the slope, p, is independent of β and hardens with increasing σ as p ≃ 1.8 + 0.7 / σ . Electrons are primarily accelerated by the nonideal electric field at X-points, either in the initial current layer or in current sheets generated between merging magnetic islands. At higher values of β, the electron power law steepens, and the electron spectrum eventually approaches a Maxwellian distribution for all values of σ. At values of β near βmax ≈ 1/4σ, when both electrons and protons are relativistically hot prior to reconnection, the spectra of both species display an additional component at high energies, containing a few percent of particles. These particles are accelerated via a Fermi-like process by bouncing between the reconnection outflow and a stationary magnetic island