The zeroth law of turbulence states that, for fixed energy input into large-scale motions, the statistical steady state of a turbulent system is independent of microphysical dissipation properties. This behaviour, which is fundamental to nearly all fluid-like systems from industrial processes to galaxies, occurs because nonlinear processes generate smaller and smaller scales in the flow, until the dissipation – no matter how small – can thermalise the energy input. Using direct numerical simulations and theoretical arguments, we show that in strongly magnetised plasma turbulence such as that recently observed by the Parker Solar Probe spacecraft, the zeroth law is routinely violated. Namely, when such turbulence is ‘imbalanced’ – when the large-scale energy input is dominated by Alfvénic perturbations propagating in one direction (the most common situation in space plasmas) – nonlinear conservation laws imply the existence of a ‘barrier’ at scales near the ion gyroradius. This causes energy to build up over time at large scales. The resulting magnetic-energy spectra bear a strong resemblance to those observed in situ, exhibiting a sharp, steep kinetic transition range above and around the ion-Larmor scale, with flattening at yet smaller scales. The effect thus offers a possible solution to the decade-long puzzle of the position and variability of ion-kinetic spectral breaks in plasma turbulence. The existence of the ‘barrier’ also suggests that, how a plasma is forced at large scales (the imbalance) may have a crucial influence on thermodynamic properties such as the ion-to-electron heating ratio.

中文翻译：

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关于违反空间等离子体湍流第零定律

湍流第零定律指出，对于大规模运动的固定能量输入，湍流系统的统计稳态与微观物理耗散特性无关。这种行为是从工业过程到星系的几乎所有类流体系统的基础，因为非线性过程会在流动中产生越来越小的尺度，直到耗散（无论多么小）都可以热化能量输入。使用直接的数值模拟和理论论证，我们表明在强磁化等离子体湍流中，例如最近由帕克太阳探测器航天器观测到的，第零定律经常被违反。即，当这种湍流是“不平衡的”时——当大尺度能量输入主要由向一个方向传播的阿尔芬微扰控制时（空间等离子体中最常见的情况）——非线性守恒定律意味着在离子附近的尺度上存在“屏障”回转半径。这会导致能量随着时间的推移而大规模积累。由此产生的磁能谱与观察到的非常相似原位，在离子-拉莫尔尺度之上和周围表现出尖锐、陡峭的动力学转变范围，在更小的尺度上变平。因此，这种效应为解决长达十年之久的等离子体湍流中离子动力学光谱断裂的位置和可变性之谜提供了可能的解决方案。“势垒”的存在还表明，等离子体如何在大尺度上受力（不平衡）可能对热力学性质（例如离子与电子的加热比）产生至关重要的影响。