由剪切流诱导喷流驱动的大尺度发电机
Large-Scale Dynamos Driven by Shear-Flow-Induced Jets
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中文总结 AI 辅助
本文针对剪切流诱导喷流驱动的发电机，建立解析理论并开展大规模三维模拟，揭示了平均涡度效应驱动大尺度磁场生成的机制，该机制可应用于双中子星并合等系统，为多信使天文学提供信号。
中文摘要 AI 辅助
磁场在其占据的所有尺度上都会影响诸多现象，包括恒星形成、宇宙射线输运、带电粒子加速、空间天气、行星大气中的输运以及实验室等离子体等。这些磁场通常由湍流通过一种称为发电机的过程产生并维持。1955年，E.N.帕克对小尺度湍流的效应进行参数化，提出了平均场发电机理论。这一广泛应用的理论能够再现观测到的大尺度磁场，但存在参数调谐困难的问题，因为这些参数无法从第一性原理得到证明：对湍流的研究表明，纠缠的磁场会因剪切流的拉伸作用而被折叠和破碎成小尺度结构。在此，我们考虑一种不稳定且受驱动的剪切流，建立了解析理论，并对湍流进行了三维（3D）高级计算机模拟，模拟的网格规模达到4096×4096×8192，展示了从第一性原理出发生成准周期大尺度磁场的过程。这种生成过程通过平均涡度效应实现——这是1990年提出的一种额外的平均场发电机过程。该发电机的关键在于预先生成大尺度三维喷流，这些喷流作为磁流体力学方程的拓扑保护且精确的非线性解而稳健产生。这种喷流驱动的发电机适用于剪切驱动的实验室和天体物理系统，包括双中子星并合系统，据报道，其中的发电机可能以微秒级的时标运行，在毫秒级时间内产生宇宙中最强的一些磁场，为多信使天文学提供信号。
英文摘要
At every scale they occupy, magnetic fields affect various phenomena, including star formation, cosmic ray transport, charged particle acceleration, space weather, transport in planetary atmospheres, and laboratory plasmas. These fields are often generated and sustained by turbulent flows in a process called the dynamo. In 1955, E. N. Parker parameterized the effects of small-scale turbulence to propose a mean-field dynamo theory. The widely used theory reproduces observed large-scale fields but suffers from difficulty in tuning parameters as they are not justified from first principles: Studies of turbulent flows show tangled magnetic fields, which are folded and fragmented into small-scale structures due to shear-flow straining. Here, considering a shear flow that is unstable and driven, we develop analytic theory and perform three-dimensional (3D), advanced computer simulations of turbulence with up to 4096 x 4096 x 8192 grid points, showing ab initio generation of quasi-periodic, large-scale magnetic fields. The generation occurs via the mean-vorticity effect---an additional mean-field dynamo process postulated in 1990. Crucial to this dynamo is the prior generation of large-scale 3D jets, robustly produced as topologically protected and exact nonlinear solutions of the magnetohydrodynamic equations. The jet-driven dynamo applies to shear-driven laboratory and astrophysical systems. These include binary neutron star mergers, where the reported dynamo likely operates on microsecond timescales to produce in milliseconds some of the strongest magnetic fields in the Universe, providing signals for multimessenger astronomy.
