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Scaling of magnetic dissipation and particle acceleration in ABC fields
Journal of Plasma Physics ( IF 2.5 ) Pub Date : 2021-05-03 , DOI: 10.1017/s0022377821000209
Qiang Chen , Krzysztof Nalewajko , Bhupendra Mishra

Using particle-in-cell numerical simulations with electron–positron pair plasma, we study how the efficiencies of magnetic dissipation and particle acceleration scale with the initial coherence length $\lambda _0$ in relation to the system size $L$ of the two-dimensional ‘Arnold–Beltrami–Childress’ (ABC) magnetic field configurations. Topological constraints on the distribution of magnetic helicity in two-dimensional systems, identified earlier in relativistic force-free simulations, that prevent the high- $(L/\lambda _0)$ configurations from reaching the Taylor state, limit the magnetic dissipation efficiency to about $\epsilon _{\textrm {diss}} \simeq 60\,\%$ . We find that the peak growth time scale of the electric energy $\tau _{E,{\textrm {peak}}}$ scales with the characteristic value of initial Alfvén velocity $\beta _{A,{\textrm {ini}}}$ like $\tau _{E,\textrm {peak}} \propto (\lambda _0/L)\beta _{A,{\textrm {ini}}}^{-3}$ . The particle energy change is decomposed into non-thermal and thermal parts, with non-thermal energy gain dominant only for high initial magnetisation. The most robust description of the non-thermal high-energy part of the particle distribution is that the power-law index is a linear function of the initial magnetic energy fraction.

中文翻译:

ABC 场中磁耗散和粒子加速度的标度

使用电子 - 正电子对等离子体的粒子单元数值模拟,我们研究了磁耗散和粒子加速的效率如何随初始相干长度变化 $\lambda_0$ 与系统大小有关 $L$ 二维“Arnold-Beltrami-Childress”(ABC)磁场配置。二维系统中磁螺旋度分布的拓扑约束,早期在相对论无力模拟中确定,防止高- $(L/\lambda _0)$ 从达到泰勒状态的配置,将磁耗散效率限制在大约 $\epsilon _{\textrm {diss}} \simeq 60\,\%$ . 我们发现电能的峰值增长时间尺度 $\tau _{E,{\textrm {峰值}}}$ 与初始 Alfvén 速度的特征值成比例 $\beta _{A,{\textrm {ini}}}$ 喜欢 $\tau _{E,\textrm {峰值}} \propto (\lambda _0/L)\beta _{A,{\textrm {ini}}}^{-3}$ . 粒子能量变化被分解为非热和热部分,非热能增益仅在高初始磁化时占主导地位。对粒子分布的非热高能部分最可靠的描述是幂律指数是初始磁能分数的线性函数。
更新日期:2021-05-03
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