Solar, stellar and planetary astrophysics in DAMTP

太阳的磁场可以通过太阳黑子的出现在表面，这也与太阳耀斑和日珥有关。太阳黑子的活动不是恒定的，而是以11年为周期的盛衰。混乱的磁场可以通过构成太阳外部的等离子体的湍流运动来维持，但周期性的行为显示了两个半球之间的一致性，并且显然是一个贯穿整个对流区的全球性过程。目前还不清楚磁场是如何组织自己产生如此大规模的周期性行为的。一种自然的大规模效应是由太阳的内部旋转提供的，它在对流区的底部附近变化很快，大约是距离太阳表面三分之二的距离。我们的工作致力于建立一个周期性活动的模型，该模型从这种剪切运动中汲取能量，并通过磁浮力机制产生上升的磁场结构，最终以太阳黑子的形式出现。结合对流效应的简化描述，该模型将得到数值解决，预计将导致具有可与太阳行为相比较的大规模特征的自我维持磁场。围绕中心质量旋转的物质盘在许多天文环境中都有发现，包括围绕年轻恒星的尘埃气体的原行星盘，行星形成的地方，黑洞周围的高能等离子体吸积盘，以及更熟悉的例子，如土星环和螺旋星系。在其他恒星周围不断发现各种各样的行星和行星系。我们建议研究天体物理盘动力学，行星形成物理学和太阳系外行星系统动力学的几个方面。我们将研究圆盘中的湍流、磁场和漩涡的性质，以及非圆形和扁平圆盘的行为。我们将研究系外行星与其宿主恒星之间的潮汐相互作用，这种相互作用可以强烈加热甚至摧毁行星，以及行星与盘的相互作用，这种相互作用可以极大地改变行星轨道的大小和形状。所有这些工作都与目前的观察有关。太阳物理学中一个突出的问题是了解太阳日冕是如何被加热的。我们知道，磁场在从太阳表面向太阳大气层输送和转移能量方面起着关键作用。这种情况发生在许多尺度上，从纳米耀斑到微耀斑、大耀斑、日珥爆发和日冕物质抛射。然而，我们还没有完全理解磁能是如何转化为热能和动能的。最近的观测表明，太阳大气是高度动态的；成像仪器（SoHO/EIT， TRACE, Stereo， Hinode/XRT以及最近的SDO/AIA）提供了壮观的高空间分辨率图像和高节奏的电影。这表明平衡模型可能不合适，非平衡效应可能需要重新审视，例如瞬态电离和重组以及非麦克斯韦电子分布。EUV（和x射线）光谱学与原子物理计算相结合，在太阳物理领域发挥着重要作用。它使等离子体的物理参数（温度和电子密度分布、流动、元素丰度和非热增宽）得以确定，并对各种加热模型施加限制。我们第一次有了来自SOHO、Hinode和SDO卫星的足够详细的光谱观测，我们可以直接将观测到的数量与理论模型预测的数量进行比较，至少是在日冕环和耀斑方面。此外，我们第一次可以将日冕特性与出现时在光球层中观察到的磁场演变联系起来。

The Sun's magnetic field can be seen at the surface through the appearance of sunspots, which are also associated with solar flares and prominences. Sunspot activity is not constant, but waxes and wanes on an 11-year timescale. Disordered magnetic fields can be maintained by the turbulent motions of the plasma making up the outer part of the Sun, but the cyclical behaviour shows coherence between the two hemispheres and is clearly a global process operating throughout this convective zone. It is not clearly understood how the magnetic field organizes itself to produce such large-scale cyclical behaviour. A natural large-scale effect is provided by the internal rotation of the Sun, which varies rapidly near the base of the convection zone about two-thirds of the distance to the surface. Our work is devoted to producing a model of cyclical activity that draws its energy from this shearing motion and produces the rising magnetic field structures that eventually emerge as sunspots through the mechanism of magnetic buoyancy. Together with a simplified description of the effect of the convection, this model, which will be solved numerically, is expected to lead to a self-sustaining magnetic field with large-scale features that can be compared with solar behaviour.Discs of matter orbiting around a central mass are found in numerous astronomical settings, including protoplanetary discs of dusty gas surrounding young stars, where planets are formed, high-energy plasma accretion discs around black holes, and more familiar examples such as Saturn's rings and spiral galaxies. A great variety of planets and planetary systems continue to be discovered around other stars. We propose to investigate several aspects of the dynamics of astrophysical discs, the physics of planet formation and the dynamics of extrasolar planetary systems. We will study the properties of turbulence, magnetic fields and vortices in discs, and the behaviour of discs that are not circular and flat. We will investigate the tidal interaction between extrasolar planets and their host stars, which can strongly heat or even destroy the planets, and the interaction of planets and discs, which can greatly modify the size and shape of the planets' orbits. All this work is related to current observations.One of the outstanding problems in solar physics is to understand how the solar corona is heated. We know that the magnetic field plays a key role in transporting and transferring energy from beneath the solar surface into the solar atmosphere. This happens on many scales from nanoflares to microflares, major flares, prominence eruptions and coronal mass ejections. However, we do not yet fully understand how magnetic energy is converted into thermal and kinetic energy. Recent observations show that the solar atmosphere is highly dynamic; imaging instruments (SoHO/EIT, TRACE, Stereo, Hinode/XRT and more recently SDO/AIA) have provided spectacular high-spatial-resolution images and high-cadence movies. These suggest that equilibrium models may not be appropriate and non-equilibrium effects may need to be revisited, for example transient ionization and recombination and non-Maxwellian electron distributions.EUV (and X-ray) spectroscopy, combined with atomic physics calculations, is playing a major role in the field of solar physics. It is enabling the physical parameters of the plasma (temperature and electron density distributions, flows, elemental abundances and non-thermal broadening) to be determined and constraints to be placed on the various heating models. For the first time, we have spectroscopic observations from the SOHO, Hinode and SDO satellites detailed enough that we can directly compare observable quantities with those predicted by theoretical modelling, at least for coronal loops and flares. Also, for the first time, we can link the coronal properties with the evolution of the magnetic field as is observed in the photosphere while emerging.