Helicity and eruptivity of solar magnetic flux ropes

这项研究的目的是更好地了解太阳的日冕物质抛射。特别是，在低太阳日冕的磁通量绳的螺旋度可以用来预测他们的爆发，它与喷出的磁云的拓扑结构？背景：磁通绳是太阳大气中磁力线的扭曲束.它们可能从太阳内部以现成的形式出现，也可能通过表面流动和磁重联的结合在大气中形成。它们被观测为太阳盘面上的丝状通道，或者是边缘上方的冕腔。现在人们普遍认为爆发的通量绳是日冕物质抛射（CME）的主要来源之一。CME将大量的磁化等离子体抛入行星际空间，是地球空间天气的主要驱动力之一。通过干扰我们的磁层，这可能会对卫星通信，飞机甚至地面上的电力网络造成重大干扰。目前，我们无法预测日冕物质抛射，直到它们离开太阳。即使这样，我们也无法预测它们的地球效应，直到（如果）它们通过我们的近地卫星之一，因为这取决于CME的内部磁结构。最近，Yeates和Hornig发展了一种称为场线螺旋度的测量方法，描述了日冕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绳。本项目将以此为起点，探索磁绳结构和喷发的物理学，并着眼于预测后者。建议方法：第1年。该项目将首先开发一个二维数值代码，以笛卡尔几何模型模拟单个磁通绳的形成和喷发。其目的是（一）发展的数学模型，数值方法，并在一个简化的设置实现的理解，和（二）进行参数研究，以确定是否扭曲或通量是喷发的最佳预测在这样的磁通绳，以及覆盖的磁场的作用。换句话说，什么是即将爆发的“警告信号”？2-3年。该项目将继续考虑球面几何的3D模拟，模拟完整的日冕。这一阶段将使用现有的并行磁摩擦代码开发的耶茨，可以遵循形成的磁通绳在全球范围内，以及模拟其失去平衡。预计将在两个方向上扩展工作。首先，测试第1年研究的结果是否在具有不同形状的真实通量绳的全3D几何中成立。这里的最终目标是能够在模拟中看到通量绳，并提前预测它是否会爆发（至少有一定的概率）。即使在数值模拟中，这在以前也没有实现。其次，要确定有多少喷发前磁力线螺旋度喷发通量绳喷射通过外部模型边界，有多少是简单地失去了通过周围的日冕磁场在喷发过程中的重新配置。确定实际喷出的结构与空间天气预测高度相关，可以用最新的工具进行新的研究。

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.