arXiv 2304.030232023-04physics.plasm-phastro-ph.HE

粒子在 MHD 湍流中通过与磁场急剧弯曲的局域相互作用输运

Particle transport through localized interactions with sharp magnetic field bends in MHD turbulence

Martin Lemoine

备注：: version published in J. Plasma Phys. Lett
DOI：: 10.1017/S0022377823000946

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AI总结：

研究 MHD 湍流中磁场急剧弯曲对粒子输运的影响，发现局域剧烈相互作用导致磁矩扩散，平行平均自由程按 $r_{\rm g}^{0.3}\ell_{\rm c}^{0.7}$ 缩放，输运过程为非布朗型。

摘要：

当粒子穿过磁场曲率半径缩小至其回旋半径 $r_{\rm g}$ 以下的区域时，会经历非绝热（破坏磁矩）的俯仰角变化。本文将该观察引入磁流体力学（MHD）湍流，研究磁场线间歇性急剧弯曲对粒子输运的影响。基于不可压缩湍流模拟中的专门测量，论证了在所有尺度上存在足够数量的大曲率区域以促进散射。由曲率强度的幂律统计预测的平行平均自由程按 $r_{\rm g}^{0.3}\\,\ell_{\rm c}^{0.7}$（$\ell_{\rm c}$ 为湍流相干尺度）比例缩放，这对宇宙线现象学具有直接意义。该数值模拟中的粒子追踪证实，磁矩通过局域剧烈相互作用发生扩散，与上述图景一致。相应地，整体输运过程在直至 $\gtrsim\ell_{\rm c}$ 的长度尺度上是非布朗的。

英文摘要：

When a particle crosses a region of space where the curvature radius of the magnetic field line shrinks below its gyroradius $r_{\rm g}$, it experiences a non-adiabatic (magnetic moment violating) change in pitch angle. The present paper carries that observation into magnetohydrodynamic (MHD) turbulence to examine the influence of intermittent, sharp bends of the magnetic field lines on particle transport. On the basis of dedicated measurements in a simulation of incompressible turbulence, it is argued that regions of sufficiently large curvature exist in sufficient numbers on all scales to promote scattering. The parallel mean free path predicted by the power-law statistics of the curvature strength scales in proportion to $r_{\rm g}^{0.3}\,\ell_{\rm c}^{0.7}$ ($\ell_{\rm c}$ coherence scale of the turbulence), which is of direct interest for cosmic-ray phenomenology. Particle tracking in that numerical simulation confirms that the magnetic moment diffuses through localized, violent interactions, in agreement with the above picture. Correspondingly, the overall transport process is non-Brownian up to length scales $\gtrsim\ell_{\rm c}$.

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