非螺旋二维无碰撞磁湍流中的逆转移：具有动力学效应的岛合并图像
Inverse Transfer in Non-helical 2D Collisionless Magnetic Turbulence: Island-Merger Picture with Kinetic Effects
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中文总结 AI 辅助
研究无碰撞等离子体形成的衰减非螺旋湍流中的磁逆转移，用粒子模拟发现其与MHD情况不同，如衰减指数及谱峰迁移情况，指出天体物理无碰撞等离子体中磁相干可通过逆转移增长，MHD衰减时间尺度外推可能高估大尺度场增长速率。
中文摘要 AI 辅助
磁逆转移常被用于将高能和宇宙等离子体中的小尺度磁场生成与更大的相干尺度联系起来。潜在的磁流体动力学（MHD）观点结合了两个逻辑上不同的要素：在小电阻率极限下渐近守恒的体数量，以及由衰减动力学确定的时间尺度。在这项工作中，我们使用粒子模拟探索这种情况在由无碰撞等离子体形成的衰减非螺旋湍流中是否仍然成立。模拟大致满足如MHD情况中的$B^2\xi_B^2\simeq{\rm const}$，并且在$B^2\propto t^{-p}$和$\xi_B\propto t^q$中的拟合指数服从$p\simeq2q$。然而，从衰减指数推断出的衰减时间尺度与MHD情况不同。我们发现在所有具有不同初始磁化强度$\sigma_0$的情况下，$p<1$且$q<1/2$，两个指数都低于MHD值并随$\sigma_0$系统变化。谱峰也以比$\xi_B$增长更快的速率向更低波数迁移，表明自相似性被破坏。自相似性的破坏归因于压力各向异性和拉莫尔尺度磁结构导致磁能谱中出现动力学尺度。这些结果表明，在天体物理无碰撞等离子体中，包括但不限于太阳风、脉冲星风星云、星际介质和宇宙等离子体，磁相干可以通过逆转移继续增长，但基于MHD衰减时间尺度的外推可能高估大尺度场的增长速率。
英文摘要
Magnetic inverse transfer is often invoked to connect small-scale magnetic-field generation to larger coherence scales in high-energy and cosmological plasmas. The underlying magnetohydrodynamic (MHD) arguments combine two logically distinct ingredients: a bulk quantity that is asymptotically conserved in the limit of small resistivity, and a time scale determined by the decay dynamics. In this work, we explore whether this scenario still holds in decaying nonhelical turbulence formed by collisionless plasmas using particle-in-cell simulations. The simulations approximately satisfy $B^2ξ_B^2\simeq{\rm const}$ as in the MHD case, and the fitted exponents in $B^2\propto t^{-p}$ and $ξ_B\propto t^q$ obey $p\simeq2q$. Here $B^2\equiv\langle B_x^2+B_y^2\rangle$ is the average in-plane magnetic energy density, and $ξ_b$ is the magnetic integral scale. However, the decay time scale differs from the MHD case as inferred from the decay exponents. We found $p<1$ and $q<1/2$ in all cases with different initial magnetization $σ_0$, with both exponents lower than the MHD values and varying systematically with $σ_0$. The spectral peak also migrates toward lower wavenumber at a rate faster than the growth of $ξ_B$, indicating a broken self-similarity. The broken self-similarity is attributed to the appearance of kinetic scales in the magnetic energy spectrum due to pressure anisotropy and Larmor-scale magnetic structures. These results indicate that in astrophysical collisionless plasmas, including but not restrict to solar wind, pulsar-wind nebulae, interstellar medium, and cosmological plasmas, magnetic coherence can continue to grow by inverse transfer, but extrapolations based on MHD decay-time scaling can overestimate the rate of large-scale field growth.
