Helicity and eruptivity of solar magnetic flux ropes

这项研究的目的是为了更好地了解太阳的日冕物质抛射。特别是，低日冕磁通量绳的螺旋度是否可以用来预测它们的喷发，以及它是否与喷出的磁云的拓扑结构相关？背景：磁通绳是太阳大气层中扭曲的磁力线束。它们可能从太阳内部形成，也可能通过表面流动和磁重联的结合在大气中形成。它们被观察为太阳盘上的细丝通道，或在太阳翼上的日冕洞。现在人们普遍认为，喷发的通量绳是日冕物质抛射（cme）的主要来源之一。日冕物质抛射大量磁化等离子体进入行星际空间，是地球上太空天气的主要驱动因素之一。通过扰乱我们的磁层，这可能会对卫星通信、飞机甚至地面电网造成重大干扰。目前，我们无法预测日冕物质抛射，直到它们离开太阳。即便如此，我们也无法预测它们的地球效应，除非它们经过我们的近地卫星，因为这取决于日冕物质抛射的内部磁结构。最近，Yeates & Hornig开发了一种称为场线螺旋度的测量方法，它描述了太阳日冕中磁场的局部拓扑结构。Lowder & Yeates随后进行了一项开创性的研究，表明这种测量方法可以用于在日冕磁场的全球模拟中识别磁通量绳。本项目将以此为起点，探索通量绳结构和喷发的物理性质，并着眼于预测后者。建议方法：第一年。该项目将首先开发一个二维数值代码，以笛卡尔几何形式模拟单个磁通量绳的形成和喷发。其目的是：(i)发展对数学模型、数值方法及其在简化设置中的实现的理解，以及（ii）进行参数研究，以确定捻度或通量是这种通量绳中喷发的最佳预测因子，以及上覆磁场的作用。换句话说，火山爆发的“警告信号”是什么？年2 - 3。该项目将继续考虑球面几何的3D模拟，模拟整个太阳日冕。这一阶段将使用Yeates开发的现有的平行磁摩擦代码，该代码可以在全球范围内跟踪通量绳的形成，并模拟它们的平衡损失。预期将在两个方向上扩大工作。首先，测试一年级的研究结果是否在完全三维几何形状与不同形状的现实通量绳持有。这里的最终目标是能够在模拟中查看通量绳，并提前预测它是否会爆发（至少，有一些附加的概率）。即使在数值模拟中，这一点以前也没有实现过。其次，确定喷发通量绳的喷发前场线螺旋度中有多少是通过外模型边界喷射出来的，有多少是在喷发期间通过周围日冕磁场的重新配置而损失的。确定实际喷出的结构与空间天气预报高度相关，可以用最近的工具进行新研究。

The aim of this study is to better understand coronal mass ejections from the Sun. In particular, can the helicity of magnetic flux ropes in the low solar corona be used to predict their eruption, and does it correlate with the topologies of the ejected magnetic clouds? Background: Magnetic flux ropes are twisted bundles of magnetic field lines in the solar atmo- sphere. They may emerge ready-formed from the solar interior, or may form in the atmosphere through a combination of surface flows and magnetic reconnection. They are observed as fila- ment channels on the solar disk, or as coronal cavities above the limb. There is now general agreement that erupting flux ropes are one a major source of coronal mass ejections (CMEs). Throwing vast quantities of magnetised plasma into interplanetary space, CMEs are one of the major drivers of space weather here at Earth. By disturbing our magnetosphere, this can cause major disruption to satellite communications, aircraft, and even power networks on the ground. At present, we are unable to predict CMEs until they leave the Sun. Even then, we are unable to predict their geo-effectiveness until (and if) they pass one of our near-Earth satellites, since this depends on the CME's internal magnetic structure. Recently, Yeates & Hornig have developed a measure called field-line helicity that describes the local topology of the magnetic field in the solar corona. Lowder & Yeates then carried out a pioneering study showing that this measure can be used to identify magnetic flux ropes in global simulations of the coronal magnetic field. This project will take this as a starting point to probe the physics of flux rope structure and eruptivity, with an eye to predicting the latter. Proposed methodology: Year 1. The project will start by developing a 2D numerical code to model the formation and eruption of a single magnetic flux rope, in Cartesian geometry. The purposes are (i) to develop an understanding of the mathematical model, numerical methods, and their implementation in a simplified setting, and (ii) to carry out a parameter study to determine whether twist or flux is the best predictor of eruptivity in such a flux rope, as well as the role of the overlying magnetic field. In other words, what are the "warning signs" of an impending eruption? Years 2-3. The project will move on to consider 3D simulations in spherical geometry, modelling the full solar corona. This stage will use an existing parallel magneto-frictional code developed by Yeates, that can follow the formation of flux ropes in the global context, as well as modelling their loss of equilibrium. It is envisaged to extend the work of in two directions. Firstly, to test whether the results of the Year 1 study hold in fully 3D geometry with realistic flux ropes of varying shapes. The ultimate goal here is to be able to look at a flux rope in the simulation and predict ahead of time whether or not it will erupt (at least, with some attached probability). Even in numerical simulations, this has not previously been achieved. Secondly, to determine how much of the pre-eruption field-line helicity of erupting flux rope is ejected through the outer model boundary, and how much is simply lost through reconfiguration of the surrounding coronal magnetic field during the eruption. Determining the structure that is actually ejected is highly relevant for space weather prediction, and can be newly studied with recent tools.