Magnetic Field Evolution in Astrophysics

磁场在宇宙中随处可见，并在宇宙中一些能量最强的现象中发挥着关键作用。这包括加速高能粒子产生宇宙射线，以及调节吸积过程中角动量的传输--帮助推动恒星的形成和类星体的光度。这一提议旨在了解宇宙中大尺度磁场的产生及其演化所涉及的两个基本物理问题，即磁重联和磁发电机。磁重联是指磁力线切换连接或相互穿透的过程。在完全导电性的极限下，这应该是不可能的，这是大多数天体物理等离子体的合理近似值。太阳观测表明，这不仅是可能的，而且非常迅速，尽管不是以一种平稳持续的方式。我将继续一系列研究，通过观察中性气体的影响和了解经历大规模重联的区域的详细性质，确定湍流为快速磁重联提供了充分的条件。我还将研究一些情况，在这些情况下，即使在没有湍流和等离子体效应的情况下，重新连接也可以快速进行，这是由非平凡的拓扑驱动的。这项工作的另一个主要目的是理解大尺度磁场的产生，即包含磁场的物体的典型尺度，而不是这些物体内湍流的涡流大小。在恒星和星系中都可以看到如此大规模的磁场，而且这两种情况下的演化速度似乎都相对较快--恒星演化数年，星系可能演化10亿年。磁螺旋度的守恒，也就是磁场的“扭曲”，已被证明有力地限制了这些磁场的增长。我将使用分析工作和模拟来展示恒星和星系等差异旋转系统中的湍流如何自然地导致绕过这些限制的内部磁螺旋度流动。

Magnetic fields are found everywhere in the universe, and play a critical role in some of the most energetic phenomena in the universe. This includes accelerating high energy particles to make cosmic rays and mediating the transport of angular momentum in accretion - helping drive star formation and the luminosity of quasars. This proposal is aimed at understanding two of the basic physical issues involved in the generation of large scale magnetic fields in the universe and their evolution, magnetic reconnection and the magnetic dynamo. Magnetic reconnection refers to the process by which magnetic field lines switch their connections, or pass through one another. In the limit of perfect conductivity, which is a reasonable approximation to most astrophysical plasmas, this should be impossible. Solar observations show that it is not only possible, but extremely rapid, albeit not in a smooth continous manner. I will continue a line of research which has established that turbulence supplies a sufficient condition for rapid magnetic reconnection, by looking at the effects of neutral gas and understanding the detailed properties of regions undergoing large scale reconnection. I will also be looking at cases where reconnection can be rapid even in the absence of turbulence and plasma effects, driven by non-trivial topologies. The other major thrust of this work is understanding the generation of magnetic fields on large scales, i.e. scales typical of the object containing the field rather than the eddy size of turbulence within those objects. Such large scale fields are seen in stars and in galaxies, and in both cases appear to evolve relatively quickly - years for stars and perhaps a billion years for galaxies. The conservation of magnetic helicity, the 'twistiness' of the magnetic field, has been shown to powerfully constrain the growth of such fields. I will be using analytic work and simulations to show how turbulence in differentially rotating systems - like stars and galaxies, naturally leads to an internal flow of magnetic helicity which bypasses these constraints.