Angular momentum transport and magnetism in stars and planets

当恒星或行星内部发生运动时，它们既可以传递热量，也可以传递角动量。因为这些运动--无论是对流的、波状的，还是两者的某种混合--无处不在，角动量输运也是如此。因此，行星和恒星内部的不同部分往往以不同的速度旋转，对物质的混合、恒星和行星磁性的产生以及恒星演化的结束都产生了深远的影响。例如，大质量恒星核的自转速度可能会影响核心坍塌后产生的致密残留物的性质，并强烈影响目前正在由引力波天文学探测的黑洞的出生自转速度。最近的观测表明，我们目前对角动量输运的理论理解--从而也就是差分旋转--存在一些重大缺陷。例如，对恒星自转速度的星震测量表明，演化后的恒星核心的自转速度比简单的运输理论模型预测的要慢得多。对我们太阳系中的天体的观测也带来了重大谜团：例如，朱诺任务最近揭示，观测到的木星带状纬向流只存在于地球最外面的几个百分比，在这一百分比以下，木星将过渡到近乎固体的自转。这被广泛地解释为源于磁场对流动的反馈，观察到的向固体旋转的转变发生在大约深度，在那里电导率变得足够高，足以使磁场与运动耦合。这与太阳形成鲜明对比，太阳的传导性自始至终都很高，但仍保持着相当大的切变。不同的结果很可能反映了这两个天体中不同的流动速度和自转速度--但关于磁化对流区角动量传输的一致理论尚未出现，该理论可以详细解释这两个结果。在这个项目中，您将结合3D数值模拟、分析理论和一维建模来研究恒星和行星内部流动实现的角动量传输。在此过程中，您将研究由气流产生的磁场(这反过来也会影响动量传输)。你的确切角色在某种程度上将取决于你自己的背景和兴趣。在我们之间(即，你、我和不同的合作者)，我们的目标是在局部(笛卡尔)和全局(球面)几何图形中进行一组3D模拟，并将得到的传输与最近发展的半解析理论中设想的传输进行比较。这将涉及到使用基于埃克塞特和其他地方的大规模并行计算机。我们将使用模拟来测试和校准半解析处方，并最终尝试将这些纳入结构和进化的一维进化模型。我们也可能，更多地推测，探索热和角动量输运以及场产生的可能性，可以在演化计算的同时，基本上通过求解有限数量的空间模式的高度简化的流体方程组来自我一致地求解。这个项目将最适合对天体物理流体动力学感兴趣的人，就像应用于恒星或行星一样。它将需要一定程度的计算熟练程度，因此至少有一定的编程经验(以及对基于Unix的环境有基本的熟悉)将是有帮助的。对流体力学/MHD有一定的了解将是很好的，但不是必须的。

When motions occur inside a star or planet, they can transport both heat and angular momentum. Because these motions - whether convective, wave-like, or some mix of the two - are ubiquitous, so is angular momentum transport. As a consequence, different parts of planetary and stellar interiors often rotate at different rates, with profound consequences for the mixing of material, the generation of stellar and planetary magnetism, and the end-states of stellar evolution. The spin rates of massive stellar cores, for example, may influence the nature of the compact remnant that occurs after core collapse, and strongly affect the natal spin rates of black holes, now being probed by gravitational wave astronomy.Recent observations have revealed that our current theoretical understanding of the angular momentum transport - and hence of differential rotation - has some major shortcomings. For example, asteroseismic measurements of the rotation rates of stars have demonstrated that evolved stellar cores rotate much slower than simple theoretical models of the transport would predict. Major puzzles have come from observations of objects in our own Solar System, too: for example, the Juno mission has recently revealed that Jupiter's observed banded zonal flows persist only throughout the outermost few percent of the planet, with a transition to nearly solid-body rotation below this. This has been widely interpreted as arising from magnetic field feedbacks on the flow, with the observed transition to solid-body rotation occurring at roughly the depth where the conductivity becomes high enough for the field to couple to the motion. This is in sharp contrast to the Sun, where conductivity is high throughout and yet a substantial shear is maintained. The different outcomes likely reflect the different regimes of flow speed and rotation rate in the two objects -- but a consistent theory of angular momentum transport in magnetised convection zones that can explain both outcomes in any detail has not yet been forthcoming.In this project, you will use a combination of 3D numerical simulations, analytical theory, and 1D modeling to study the angular momentum transport achieved by flows in stellar and planetary interiors. Along the way, you will study the magnetic fields that are generated by the flows (and which in turn also affect the momentum transport). Your precise role will depend to some extent on your own background and interests. Between us (i.e., you, me, and various collaborators), we will aim to conduct a set of 3D simulations in both local (Cartesian) and global (spherical) geometries, and compare the resulting transport to that envisioned in recently-developed semi-analytical theories. This will involve using massively parallel computers based here in Exeter, and elsewhere. We will use the simulations to test and calibrate the semi-analytical prescriptions, and ultimately attempt to incorporate these into a 1D evolutionary model of structure and evolution. We may also, more speculatively, explore the possibility that the heat and angular momentum transport, and field generation, can be solved for self-consistently, in parallel with the evolutionary calculation, essentially by solving a highly simplified set of fluid equations for a finite number of spatial modes.This project would be most suitable for someone with an interest in astrophysical fluid dynamics, as applied to stars or planets. It will require some level of proficiency with computation, so at least a modest amount of programming experience (and a basic familiarity with Unix-based environments) would be helpful. Some prior familiarity with fluid dynamics/MHD would be great, but is not essential.