Reconnection Onset in Overstretched Magnetotail Current Sheets

当太阳风流过地球时，它将地球的磁力线拉长，使其远离太阳，在磁场中储存了大量的能量，同时形成了磁尾。当这种拉伸达到临界水平时，磁力线重新连接，迅速释放储存的能量，同时为重要的空间天气效应提供动力，例如磁层亚暴，这可能对卫星和地球上的电网构成危险。理解描述拉伸磁尾和重联开始的物理学对于建模和预测空间天气现象至关重要。重连接的开始需要磁尾电流片（CS）薄到等离子体粒子的尺度，如质子甚至电子轨道，这与广阔的磁尾相比几乎是微观的。本项目旨在研究这些超拉伸CSs的结构、稳定性和重联机制，以了解磁尾重联发生的机制，并解释其在近地区域之外的观测位置。该研究的数据挖掘部分将有助于研究相对较少的机器学习领域，该领域处理少量和稀疏的数据，而不是大数据。这个项目更广泛的影响包括对研究生的动手研究训练，早期职业科学家的职业发展，以及对所开发的方法的跨学科应用。磁尾重新连接的开始要求其电流片（CS）变薄到热离子陀螺半径（或更薄），以使离子（甚至电子）退磁并提供它们的朗道耗散。然而，在各向同性等离子体中，离子尺度的CSs随着离地球的距离膨胀得太快，在地球半径的20倍处（大多数x线被观测到的地方）无法保持离子尺度。由于“过度拉伸”薄CS （OTCS）的发现，最近发现了解决这个问题的关键：如果离子尺度CS嵌入到更厚的CS中，即使具有弱的场向离子各向异性，其电流密度等等值线也可以比磁力线拉伸得更多。该项目的目标是研究otcs的结构、稳定性和重联机制，以回答以下基本科学问题：磁尾重联发生的机制是什么，这解释了它在近地区域之外的观测位置，以及它对全球的影响是什么？在二维和三维PIC模拟中，将使用新的OTCS平衡来研究负电荷和电子电流支配的影响，撕裂和气球/交换模式之间的相互作用，以及外部驱动和局部磁通量积累的作用。OTCS对全球磁层结构和动力学的影响将通过将它们通过挖掘多任务和数十年的星载磁力计数据库和新一代磁场结构获得的经验重建结果添加到全球磁层MHD模型中来评估。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

As the solar wind flows past the Earth, it stretches our planet’s magnetic field lines away from the Sun, storing vast amounts of energy within the magnetic field while forming the magnetotail. When this stretching reaches a critical level, the magnetic field lines reconnect, rapidly releasing the stored energy while powering important space weather effects, such as magnetospheric substorms, that can be hazardous to satellites and the power grid on Earth. Understanding the physics describing the stretched magnetotail and reconnection onset is crucial to modeling and predicting space weather phenomena. Reconnection onset requires the magnetotail current sheet (CS) to thin down to the scales of the plasma particles, such as protons or even electrons orbits, which are nearly microscopic compared to the vast expanse of the magnetotail. This project aims to investigate the structure, stability, and reconnection regimes of these overstretched CSs to understand the mechanism of reconnection onset in the magnetotail and explain its observed location far outside the near-Earth region. The data-mining component of the research will contribute to the relatively under-investigated area of machine learning dealing with little and sparse rather than big data. The broader impacts of this project include hands-on research training for a graduate student, the career development of early career scientists, and interdisciplinary applications to the methodology developed.The onset of reconnection in the magnetotail requires its current sheet (CS) to thin down to the thermal ion gyroradius (or thinner) to demagnetize ions (or even electrons) and to provide their Landau dissipation. However, in isotropic plasmas, the ion-scale CSs inflate too rapidly with the distance from Earth to remain ion-scale at and beyond ~20 Earth’s radii, where most X-lines are observed. A key to solving this problem was recently found due to the discovery of “overstretched” thin CS (OTCS): If an ion-scale CS is embedded into a much thicker CS with even a weak field-aligned ion anisotropy, its current density iso contours can be stretched way more than the magnetic field lines. The goal of this project is to investigate the structure, stability, and reconnection regimes of OTCSs to answer the following fundamental science question: What is the mechanism of the reconnection onset in the magnetotail, which explains its observed location far outside the near-Earth region, and what are its global implications? The effects of the negative CS charging and electron current domination suggested by recent observations, interplay between tearing and ballooning/interchange modes, as well as roles of external driving and local magnetic flux accumulation, will be investigated using new OTCS equilibria in 2D and 3D PIC simulations. The impact of OTCS on the global scale structure and dynamics of the magnetosphere will be assessed by adding their empirical reconstructions obtained by mining a multi-mission and multi-decade database of spaceborne magnetometers and new-generation magnetic field architectures to a global MHD model of the magnetosphere.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.