The Dynamics of Thin Current Sheets and the Triggering of Fast Reconnection in Different Plasma Environments

不同等离子体环境中薄电流片的动力学和快速重联的触发

• 批准号: 1619611
• 负责人: Marco Velli
• 金额: $ 36.6万
• 依托单位: University of California-Los Angeles
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-07-01 至 2019-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1619611&HistoricalAwards=false
• 关键词: Dynamics; Thin; Current; Sheets; Triggering

This research aims to understand the triggering mechanism for some of the most energetic events and beautiful light shows in the visible universe. Space and astrophysical plasmas - the ionized gases constituting the greatest part of the visible (baryonic) Universe - are responsible for a variety of observed energetic phenomena from pulsar gamma-ray flares, to solar flares, to magnetospheric substorms leading to Aurora Borealis. All of these processes are characterized by a preliminary phase of energy storage, where magnetic field energy is built up due to rotational, gravitational, or convective motions of the plasma, followed by the sudden triggering of rapid energy release. Magnetic reconnection, the splicing and reforming of magnetic field lines is thought to be at the heart of most observed explosive phenomena. The results of this research will allow a better understanding of the transition from stability to the sudden release of magnetic energy, with the ability to predict the critical parameters necessary for the transition in different astrophysical, as well as laboratory plasmas. This research project?s broader impact includes a more advanced understanding of this decades-old plasma physics problem both in laboratory and astrophysical contexts, with applications involving future laboratory experiments and advanced numerical computations, as well as NASA missions.The classic picture of magnetic reconnection involves current sheets, assumed to be planar-like and concentrated very narrowly in the third dimension. Recently, the slow, stationary reconnection scenario was transformed by the discovery that such configuration is unstable to tearing at large values of the Lundquist number, S, leading to one promising picture (a.k.a. the plasmoid instability) of fast reconnection. The plasmoid instability of the planar current sheet has a paradoxical feature, in that the instability growth rate diverges with S. Growth rates which become arbitrarily large at high S therefore beg the question of how a system transitions from stability to instability. This difficulty with diverging growth rates was resolved recently by Pucci and Velli, who showed that a limiting current sheet inverse aspect ratio separates slow and fast reconnecting modes, a property they called "Ideal Tearing", or IT. As a consequence, fast reconnection sets in in relatively thick current sheets and all plasmoid instability scalings (number of islands etc.) require correction. Such a scenario is promising in that it not only can explain observed fast reconnection rates, but might also account for the reconnection trigger mechanism. The present research program builds on the IT reconnection framework and generalizes it to different plasma configurations in different regimes by: 1) extending the linear scaling theory to more general equilibria and two dimensions including flows and kinetic regimes; 2) simulating collapsing current sheet in 3D resistive MHD to study nonlinear evolution in configurations with and without initial flows. The results will impact the understanding of catastrophic energy release in natural and laboratory plasmas.

这项研究旨在了解可见宇宙中一些最具活力的事件和美丽的光表演的触发机制。空间和天体物理等离子体 - 构成可见（重子）宇宙最大部分的电离气体 - 是造成各种观测到的高能现象的原因，从脉冲星伽马射线耀斑到太阳耀斑，再到导致北极光的磁层亚暴。 所有这些过程的特点都是能量存储的初步阶段，其中由于等离子体的旋转、重力或对流运动而建立磁场能量，然后突然触发快速能量释放。磁重联、磁力线的拼接和重组被认为是大多数观察到的爆炸现象的核心。 这项研究的结果将有助于更好地理解磁能从稳定到突然释放的转变，并能够预测不同天体物理以及实验室等离子体中转变所需的关键参数。该研究项目的更广泛影响包括在实验室和天体物理背景下对这个已有数十年历史的等离子体物理问题有了更深入的理解，其应用涉及未来的实验室实验和高级数值计算以及 NASA 任务。磁重联的经典图片涉及当前的片材，假设是平面状的并且非常狭窄地集中在三维空间中。最近，人们发现这种结构在伦德奎斯特数 S 值较大时不稳定，无法撕裂，从而改变了缓慢、稳定的重连场景，从而产生了一种有希望的快速重连图景（也称为等离子体不稳定性）。 平面电流片的等离子体不稳定性具有一个矛盾的特征，即不稳定性增长率随 S 的变化而变化。在高 S 时增长率变得任意大，因此回避了系统如何从稳定过渡到不稳定的问题。 Pucci 和 Velli 最近解决了增长率不同的难题，他们证明了限制电流片材逆纵横比可将慢速和快速重新连接模式分开，他们将这种特性称为“理想撕裂”或 IT。因此，在相对较厚的电流片中出现快速重新连接，并且所有等离子体不稳定性尺度（岛的数量等）都需要校正。这种场景很有希望，因为它不仅可以解释观察到的快速重连率，而且还可以解释重连触发机制。目前的研究计划建立在IT重联框架的基础上，并通过以下方式将其推广到不同状态下的不同等离子体配置：1）将线性标度理论扩展到更一般的平衡和二维，包括流动和动力学状态； 2) 模拟 3D 电阻 MHD 中的塌陷电流片，以研究具有和不具有初始流动的配置中的非线性演化。这些结果将影响对自然和实验室等离子体中灾难性能量释放的理解。