Multi-scale modelling of heating and particle acceleration in twisted magnetic fields in solar flares and coronal heating

太阳耀斑是剧烈而复杂的事件，它释放出几乎所有波段的电磁辐射，并直接向太空发射高能粒子。它们本身就很有趣，因为它们是发生在整个宇宙中的基本物理过程的例子，而且因为它们通过“空间天气”对地球的空间环境产生重大影响。耀斑产生的高能粒子和电磁辐射会破坏卫星和地球上的电力系统，对宇航员也有潜在的危险。已经确定的主要能量释放机制是磁重联过程。然而，仍有一些重大的突出问题有待解决：特别是大量高能（非热）离子和电子的起源问题。虽然最近的观测，特别是硬x射线成像望远镜RHESSI对这些粒子的特性有了很多新的认识，但新的观测也对理论和建模提出了新的挑战。所涉及的长度尺度范围很大——从全球尺度的百万米到基本等离子体尺度的米——使得建模成为一项特别困难的任务，没有一个模型可以涵盖所有特征。另一个长期存在的谜团是解释热x射线日冕的存在——其温度（数百万度）大大超过表面温度（几千度）。一个非常有希望的设想是，日冕加热是由许多非常小的类耀斑事件（称为纳米耀斑）的综合效应引起的。因此，基本的能量释放过程是磁重联，就像大规模的太阳耀斑一样。为了区分不同的候选日冕加热，有必要预测可观测的特征，如高能粒子的性质、温度分布和等离子体流动。扭曲的磁场提供了一个自由磁能的储存库，这些能量可以被耗散成热量，而且这种扭曲的磁场在日冕中很可能是非常常见的——无论是大尺度结构还是小尺度结构。我们之前的研究表明，如果单个扭曲的磁通绳的捻度足够大，可以迅速释放存储的磁能，从而产生理想的扭结不稳定性——这将产生小规模的碎片电流片，通过磁重联产生有效的等离子体加热和粒子加速。我们开发了一套强大的工具，将测试粒子与3D磁流体动力学模拟相结合，并对可观察到的特征（如软x射线和硬x射线发射）进行正向建模。在本项目中，我们将在此工作的基础上开发一个相互关联的层次模型，用于扭曲磁通绳中的能量释放，从更理想的二维模型到更复杂和更现实的大尺度模型。我们将开发和利用一种创新的新建模方法，称为“减少动力学”，它弥合了动力学和流体方法之间的差距。我们将利用这一方法和先进的测试粒子代码，结合磁流体动力学模拟，研究在外部干扰驱动下，等离子体加热和粒子加速在强制重联中的作用，重点研究扭曲磁通绳与重联电流片在二维和三维中的合并。我们还将在更现实的3D配置中研究热等离子体和非热等离子体，包括曲率和现实大气。除了单个不稳定回路外，我们还将探索回路之间的相互作用，特别是最近发现的“雪崩”过程，其中一个不稳定回路可能触发许多稳定邻居的能量释放。包括微波发射在内的可观测特征将被预测，这样就可以根据观测结果对不同的情景进行比较和测试。

Solar flares are dramatic and complex events, which give off electromagnetic radiation in almost all wavelength bands across the spectrum, and also directly emit high energy particles into space. They are of great interest in their own right, as examplars of fundamental physical processes which take place across the universe - and because of their significant effects on the Earth's space environment through "space weather". The high-energy particles and electromagnetic radiation from flares can damage satellites as well as power systems on the Earth, and are potentially hazardous to astronauts. It is well-established that the primary energy release mechanism is the process of magnetic reconnection. However, there are major outstanding issues to be resolved: in particular, the origin of the large numbers of high energy (non-thermal) ions and electrons. Whilst much new light has been shed on the properties of these particles by recent observations, especially from the Hard X-ray imaging telescope RHESSI, new observations have also posed new challenges to theory and modelling. The vast range of length scales involved - from the global scales of mega-metres down to fundamental plasma scale lengths of metres - makes modelling a particularly difficult task, and no single model can encompass all features.Another long-standing mystery is to explain the existence of a hot X-ray corona - whose temperature (millions of degrees) greatly exceeds the surface temperature (a few thousand degrees). One very promising scenario is that coronal heating arises from the combined effect of many very small flare-like events, known as nanoflares. Thus, the fundamental energy release process is magnetic reconnection, as in larger scale solar flares. In order to distinguish between different candidates for coronal heating, it is necessary to predict observable signatures, such as the properties of energetic particles, the temperature distribution, and plasma flows.Twisted magnetic fields provide a reservoir of free magnetic energy which could be dissipated into heating, and such twisted fields are likely to be very common in the solar corona - both as large-scale structures and on smaller scales. We have previously shown that single twisted flux ropes may rapidly release stored magnetic energy if their twist is sufficiently large for onset of the ideal kink instability - this generates small-scale fragmented currents sheets, with efficient plasma heating and particle acceleration through magnetic reconnection. We have developed a powerful set of tools, coupling test-particles to 3D magnetohydrodynamic simulations, and forward-modelling observable signatures such as soft and hard X-ray emission.In this project, we will build on this work to develop an interlinked hierarchy of models for energy release in twisted magnetic flux ropes, from more idealised 2D models to complex and more realistic larger-scale models. We will develop and exploit an innovative new modelling approach called "reduced kinetics" which bridges the gap between kinetic and fluid approaches. We will use this, and advanced test-particle codes coupled with magnetohydrodynamic simulations, to study both plasma heating and particle acceleration in forced reconnection, driven by an external disturbance, focussing on the merger of twisted flux ropes with the reconnecting current sheet in both 2D and 3D.We will also investigate thermal and non-thermal plasma in more realistic 3D configurations, including curvature and a realistic atmosphere. As well as single unstable loops, we will explore interactions between loops, especially a recently-discovered "avalanche" process whereby one unstable loop may trigger energy release from many stable neighbours. Observable signatures, including microwave emission, will be predicted, so that different scenarios can be compared and tested against observations.