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

太阳耀斑和日冕加热扭曲磁场中的加热和粒子加速的多尺度建模

基本信息

批准号：
ST/P000428/1
负责人：
Philippa Browning
金额：
$ 45.34万
依托单位：
University of Manchester
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2017
资助国家：
英国
起止时间：
2017 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=ST%2FP000428%2F1
关键词：
Multi scale modelling heating particle

项目摘要

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.

太阳耀斑是一种戏剧性且复杂的事件，它会发出整个光谱中几乎所有波长带的电磁辐射，并直接向太空发射高能粒子。它们本身就引起了人们极大的兴趣，因为它们作为宇宙中发生的基本物理过程的例子，并且因为它们通过“太空天气”对地球空间环境产生了重大影响。耀斑产生的高能粒子和电磁辐射会损坏卫星以及地球上的电力系统，并对宇航员造成潜在危险。众所周知，主要的能量释放机制是磁重联过程。然而，还有一些重大的悬而未决的问题需要解决：特别是大量高能（非热）离子和电子的起源。虽然最近的观测，特别是硬 X 射线成像望远镜 RHESSI 的观测，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。涉及的长度尺度范围广泛——从兆米的全球尺度到米的基本等离子体尺度长度——使得建模成为一项特别困难的任务，并且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）。一种非常有希望的情况是，日冕加热是由许多非常小的耀斑类事件（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。为了区分日冕加热的不同候选者，有必要预测可观测的特征，例如高能粒子的特性、温度分布和等离子体流。扭曲磁场提供了可以消散到加热中的自由磁能的储存库，并且这种扭曲场在日冕中可能非常常见 - 无论是在大尺度结构还是在较小尺度上。我们之前已经证明，如果单股扭转磁通绳的扭转足够大，足以引发理想的扭结不稳定性，那么单股扭转的磁通绳可能会快速释放存储的磁能——这会产生小规模的碎片电流片，并通过磁重联实现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，并对可观察的特征（例如软和硬 X 射线发射）进行正向建模。在这个项目中，我们将在此工作的基础上开发一个相互关联的模型层次结构，用于扭曲磁通绳中的能量释放，从更理想化的 2D 模型到复杂且更现实的大型模型。我们将开发和利用一种称为“简化动力学”的创新建模方法，它弥合了动力学方法和流体方法之间的差距。我们将使用这个以及先进的测试粒子代码与磁流体动力学模拟相结合，研究由外部扰动驱动的强制重连中的等离子体加热和粒子加速，重点关注扭曲磁通绳与重连电流片在 2D 和 3D 中的合并。我们还将研究更真实的 3D 配置中的热和非热等离子体，包括曲率和非热等离子体。现实的氛围。除了单个不稳定环路之外，我们还将探索环路之间的相互作用，特别是最近发现的“雪崩”过程，其中一个不稳定环路可能会触发许多稳定邻居的能量释放。将预测可观测的特征，包括微波发射，以便可以根据观测结果对不同的场景进行比较和测试。