Solar, stellar and planetary astrophysics in DAMTP

太阳磁场可以通过太阳黑子的出现在表面看到，太阳黑子也与太阳耀斑和日冕有关。太阳黑子活动并不是恒定不变的，而是以11年为时间尺度起伏不定。无序的磁场可以通过组成太阳外部的等离子体的湍流运动来维持，但这种周期性行为显示出两个半球之间的一致性，显然是一个在整个对流区运行的全球过程。目前还不清楚磁场是如何自我组织以产生如此大规模的周期性行为的。太阳的内部自转提供了一种自然的大尺度效应，太阳内部自转在对流区底部附近迅速变化，大约是到地表距离的三分之二。我们的工作致力于建立一个周期活动模型，该模型从这种剪切运动中获取能量，并产生上升的磁场结构，最终通过磁浮力机制出现太阳黑子。再加上对流效应的简化描述，这个将通过数值求解的模型有望产生一个自我维持的磁场，具有可以与太阳行为相比较的大范围特征。围绕中心质量旋转的物质盘在许多天文环境中都可以找到，包括年轻恒星周围尘埃气体的原行星盘(行星形成地)，黑洞周围的高能等离子体吸积盘，以及更常见的例子，如土星环和螺旋星系。在其他恒星周围继续发现各种各样的行星和行星系统。我们建议研究天体物理盘的动力学、行星形成的物理学和太阳系外行星系统的动力学的几个方面。我们将研究圆盘中的湍流、磁场和涡旋的特性，以及非圆形和平坦的圆盘的行为。我们将研究太阳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，以及行星和圆盘的相互作用，这可以极大地改变行星轨道的大小和形状。所有这些工作都与当前的观测有关。太阳物理中的一个悬而未决的问题是了解日冕是如何加热的。我们知道，磁场在将能量从太阳表面下输送和转移到太阳大气中起着关键作用。这发生在许多尺度上，从纳米耀斑到微耀斑、大耀斑、日冕喷发和日冕物质抛射。然而，我们还没有完全了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和最近的SDO/AIA)提供了壮观的高空间分辨率图像和高节奏电影。这表明平衡模型可能是不合适的，非平衡效应可能需要重新考虑，例如瞬时电离和复合以及非麦克斯韦电子分布。EUV(和X射线)光谱与原子物理计算相结合，在太阳物理领域发挥着重要作用。它能够确定等离子体的物理参数(温度和电子密度分布、流动、元素丰度和非热展宽)，并对各种加热模型施加限制。我们第一次从SOHO、Hinode和SDO卫星获得了足够详细的光谱观测，以至于我们可以直接将可观测量与理论模型预测的量进行比较，至少对于日冕环和耀斑来说是这样。此外，我们第一次可以将日冕特性与磁场的演变联系起来，就像在光球层观察到的那样。

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.