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

这项研究旨在了解可见宇宙中一些最具能量的事件和美丽的光秀的触发机制。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。 所有这些过程的特征在于能量存储的初始阶段，其中磁场能量由于等离子体的旋转、重力或对流运动而建立，随后是快速能量释放的突然触发。磁场重联，即磁力线的拼接和重组，被认为是大多数观测到的爆炸现象的核心。 这项研究的结果将使人们能够更好地理解从稳定到磁能突然释放的转变，并能够预测不同天体物理学以及实验室等离子体中转变所需的关键参数。这个研究项目？更广泛的影响包括在实验室和天体物理学背景下对这个几十年前的等离子体物理问题的更深入的理解，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。最近，缓慢的，稳定的重连场景被发现这样的配置是不稳定的撕裂在大值的伦德奎斯特数，S，导致一个有希望的图片（a.k.a.等离子体团不稳定性）的快速重联。 平面电流片的等离子体团不稳定性有一个矛盾的特征，即不稳定性增长率随S的变化而发散。因此，当增长率在高S时变得任意大时，就引出了系统如何从稳定过渡到不稳定的问题。最近，Pucci和Velli解决了这种增长率不同的困难，他们发现极限电流片的反纵横比将慢重联模式和快重联模式分开，他们称之为“理想撕裂”或IT。需要更正。这种情况是有希望的，因为它不仅可以解释观察到的快速重连率，但也可能占重连触发机制。本研究计划建立在IT重联框架的基础上，并通过以下方法将其推广到不同区域的不同等离子体构型：1）将线性标度理论扩展到更一般的平衡态和二维，包括流动和动力学区域; 2）模拟三维电阻MHD中的崩塌电流片，以研究有和没有初始流动的构型的非线性演化。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。

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.