Generation of high energy particles in solar flares - towards realistic models

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。它们之所以引起人们极大的兴趣，不仅是因为它们本身，还因为它们通过“空间天气”对地球的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破坏卫星和地球上的电力系统，对宇航员来说可能是极其危险的。19世纪，卡灵顿在白光中首次观测到耀斑，他注意到强烈的地磁活动，并推测耀斑可能与磁场有关。但是，在了解耀斑的性质和起源方面，真正的进展是在太空时代，特别是随着太空望远镜x射线观测的出现。现在已经确定，主要的能量释放机制是磁重联过程，即相反方向的磁场线被推到一起，导致存储的磁能有效耗散。磁重联发生在“电流片”——一个磁场变化强烈的区域——这是能量释放的地方。然而，有关太阳耀斑的主要突出问题有待解决：特别是大量高能（非热）带电粒子（离子和电子）的起源问题。虽然通过最近的观测，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，但这些新的观测对理论和建模提出了新的挑战。要理解耀斑中高能粒子的起源，一个非常有效的方法是计算“测试粒子”（单个质子和电子）在背景电磁场中与某种磁重联模型相对应的轨迹。这已被广泛应用于简单的二维、稳定的磁场和电场。我们建议发展这样的模型，使它们更真实，更能代表太阳耀斑的实际情况。首先，我们将考虑诸如与其他粒子碰撞和辐射发射等效应如何影响带电粒子的运动以及它们如何获得能量。其次，我们将考虑更复杂和现实的背景电磁场模型，这些模型是三维的和随时间变化的。这将包括湍流的影响，我们将探索重新连接的电流片的碎片是否可以解释在耀斑中观察到的大量质子和电子的加速。最后，我们将开始发展复杂的自洽模型，其中包括加速带电粒子对电磁场的影响。我们的理论模型的结果将与RHESSI的硬x射线观测结果以及新仪器LoFAR（低频无线电阵列）和即将到来的ALMA（阿塔卡马大型毫米阵列）的无线电和毫米波观测结果进行比较。

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 both in their own right 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 extremely hazardous to astronauts. Flares were first observed in white light during the 19th century by Carrington - who noticed strong geomagnetic activity and speculated that there might be some connection with magnetic fields. But real progress in understanding the nature and origin of flares came in the space age, particularly with the advent of observations in X-rays from space-borne telescopes. It is now well-established that the primary energy release mechanism is the process of magnetic reconnection, whereby oppositely-directed fieldlines are pushed together causing the efficient dissipation of stored magnetic energy. Magnetic reconnection occurs in a 'current sheet' - a region of strong variation in magnetic field - which is the site of the energy release. However, there are major outstanding issues concerning solar flares to be resolved: in particular, the origin of the large numbers of high energy (non-thermal) charged particles - both 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, these new observations have posed new challenges to theory and modelling. One very powerful approach to understanding the origin of high energy particles in flare is to calculate the trajectories of 'test particles' - individual protons and electrons - in background electromagnetic fields corresponding to some model of magnetic reconnection. This has been widely used for simple two-dimensional, steady magnetic and electric fields. We propose to develop such models so that they are much more realistic and representative of the actual situation in solar flares. Firstly, we will consider how effects such as collisions with other particles, and emission of radiation, affects the motion of the charged particles and how they gain energy. Secondly, we will consider much more complex and realistic models of the background electromagnetic fields, which are three-dimensional and time-dependent. This will incorporate the effects of turbulence, and we will explore whether fragmentation of the reconnecting current sheet can explain the acceleration of the large numbers of protons and electrons which are observed in flares. Finally, we will start to develop sophisticated self-consistent models which incorporate the effects of the accelerated charged particles on the electromagnetic fields. The outcomes of our theoretical models will be compared with observations of hard X-rays from RHESSI as well as radio and mm-waves from the new instruments LoFAR (Low Frequency Radio ARray) and the forthcoming ALMA (Atacama LArge Millimetre Array).