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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.

太阳耀斑是一种剧烈而复杂的事件，它发出几乎所有波段的电磁辐射，并直接向太空发射高能粒子。它们本身就具有极大的意义，因为它们是发生在整个宇宙中的基本物理过程的实例，而且还因为它们通过“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。已经确定的是，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，对这些粒子的性质有了许多新的认识，但新的观测也对理论和建模提出了新的挑战。从全球尺度的百万米到基本的等离子体尺度的米，所涉及的长度尺度范围之广，使得建模成为一项特别困难的任务，没有一个单一的模型可以涵盖所有的特征。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类似耀斑的事件（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，就像在更大规模的太阳耀斑中一样。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，例如高能粒子的性质、温度分布和等离子体流动。扭曲磁场提供了一个自由磁能的储存库，这些磁能可以被耗散成加热，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。我们之前已经表明，如果单个扭曲的磁通绳的扭曲足够大，可以快速释放储存的磁能，以启动理想的扭结不稳定性-这会产生小规模的碎片电流片，通过磁重联有效的等离子体加热和粒子加速。我们已经开发了一套强大的工具，将测试粒子耦合到3D磁流体动力学模拟，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。在这个项目中，我们将在这项工作的基础上开发一个相互关联的模型层次，用于扭曲磁通绳中的能量释放，从更理想化的2D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的建模方法，称为“减少动力学”，桥梁之间的差距差距动力学和流体的方法。我们将使用这一点，先进的测试粒子程序结合磁流体动力学模拟，研究等离子体加热和粒子加速的强制重联，由外部扰动驱动，集中在合并的扭曲磁通绳与重联电流片在2D和3D。我们还将研究热和非热等离子体在更现实的3D配置，包括曲率和现实的大气。除了单个不稳定的循环，我们还将探索循环之间的相互作用，特别是最近发现的“雪崩”过程，即一个不稳定的循环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，以便根据观测结果比较和检验不同的情景。