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，低日冕中磁通绳的螺旋度可以用来预测它们的喷发吗？它与喷射磁云的拓扑结构是否相关？背景：磁通绳是太阳大气中扭曲的磁场线束。它们可能是从太阳内部现成的，也可能是通过表面流和磁重联的结合在大气中形成的。它们被观察为太阳盘上的细丝通道，或肢体上方的冠状腔。现在人们普遍认为喷发的磁通绳是日冕物质抛射（CME）的主要来源之一。日冕物质抛射将大量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因素之一。通过干扰我们的磁层，这可能会对卫星通信、飞机甚至地面电力网络造成重大破坏。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缘效应，直到（如果）它们经过我们的一颗近地卫星，因为这取决于日冕物质抛射的内部磁结构。最近，叶茨和霍尼格开发了一种称为场线螺旋度的测量方法，它描述了日冕磁场的局部拓扑。 Lowder 和 Yeates 随后进行了一项开创性研究，表明该测量可用于识别日冕磁场全局模拟中的磁通量绳。该项目将以此为起点，探讨磁绳结构和喷发的物理原理，并着眼于预测后者。提议的方法：第一年。该项目将首先开发一个二维数字代码，以笛卡尔几何模型模拟单个磁通绳的形成和爆发。目的是（i）加深对数学模型、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扭转或通量是否是此类磁通绳中喷发性的最佳预测因素，以及上覆磁场的作用。换句话说，即将爆发的“警告信号”是什么？ 2-3年级。该项目将继续考虑球面几何的 3D 模拟，对完整的日冕进行建模。该阶段将使用 Yeates 开发的现有并行磁摩擦代码，该代码可以跟踪全局背景下磁力绳的形成，并对其失去平衡进行建模。设想将工作扩展到两个方向。首先，测试第一年研究的结果是否适用于具有不同形状的真实磁通绳的完全 3D 几何结构。这里的最终目标是能够在模拟中查看磁通绳并提前预测它是否会爆发（至少有一定的附加概率）。即使在数值模拟中，这也是以前从未实现过的。其次，确定喷发磁绳的喷发前磁力线螺旋有多少是通过外模型边界喷射出来的，以及在喷发过程中通过周围日冕磁场的重新配置而简单损失了多少。确定实际喷射的结构与空间天气预报高度相关，并且可以使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.