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

太阳耀斑是一种戏剧性的复杂事件，它在整个光谱的几乎所有波长波段都会发出电磁辐射，也会直接向太空发射高能粒子。作为整个宇宙中发生的基本物理过程的范例，它们本身具有极大的兴趣--因为它们通过“空间天气”对地球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损坏卫星和地球上的电力系统，并对宇航员构成潜在危险。众所周知，初级能量释放机制是磁重联过程。然而，还有一些重大悬而未决的问题需要解决：特别是大量高能(非热)离子和电子的来源。虽然最近的观测，特别是来自硬X射线成像望远镜RHESSI的观测，对这些粒子的性质有了许多新的了解，但新的观测也对理论和模型提出了新的挑战。涉及的长度尺度范围很广，从全球百万米尺度到基本等离子体尺度米长，使得建模变得特别困难，而且没有一个单一的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(数百万摄氏度)远远超过表面温度(几千摄氏度)。一种非常有希望的情况是，日冕加热是由许多非常小的类似耀斑的事件，即所谓的纳米耀斑的综合作用产生的。因此，基本的能量释放过程是磁重联，就像在更大范围的太阳耀斑中一样。为了区分日冕加热的不同候选者，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。扭曲磁场提供了一个自由磁能储存库，可以被耗散成加热，而这种扭曲的磁场很可能在日冕中非常常见--无论是作为大尺度的结构，还是作为小尺度的结构。我们以前已经证明，如果单个扭曲的磁通绳的扭度足够大，足以引发理想的扭结不稳定性，那么它们可能会迅速释放存储的磁能--这会产生小规模的碎裂电流片，通过磁重联有效地加热等离子体和加速粒子。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，并对软、硬X射线发射等可观测信号进行正向建模。在此项目中，我们将在此工作的基础上开发一系列相互关联的模型，用于在扭曲的磁通绳中释放能量，从更理想的2D模型到更复杂、更真实的更大尺度的模型。我们将开发和利用一种创新的新建模方法，称为“简化动力学”，它将弥合动力学方法和流体方法之间的差距。我们将使用这一方法和先进的测试粒子程序，结合磁流体模拟，研究在外部扰动驱动下强迫重联中的等离子体加热和粒子加速，重点放在2D和3D中扭曲的通量绳与重联电流片的合并。我们还将研究更现实的3D构型中的热和非热等离子体，包括曲率和真实的大气。除了单个不稳定的环路，我们还将探索环路之间的相互作用，特别是最近发现的一个不稳定环路可能触发许多稳定邻居释放能量的“雪崩”过程。包括微波发射在内的可观测特征将被预测，以便可以将不同的情景与观测进行比较和测试。

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.