Magnetic Field Evolution in Astrophysics

天体物理学中的磁场演化

• 批准号: 355457-2012
• 负责人: Vishniac, Ethan
• 金额: $ 3.72万
• 依托单位: University of Saskatchewan
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2014
• 资助国家: 加拿大
• 起止时间: 2014-01-01 至 2015-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=549731
• 关键词: Magnetic; Field; Evolution; Astrophysics

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.

磁场在宇宙中无处不在，在宇宙中一些最具活力的现象中发挥着关键作用。这包括加速高能粒子以产生宇宙射线，并在吸积过程中介导角动量的传输-帮助驱动星星的形成和类星体的亮度。 这个提议的目的是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。 磁重联是指磁场线切换它们的连接或通过彼此的过程。 在完美导电性的极限下，这是对大多数天体物理等离子体的合理近似，这应该是不可能的。 对太阳的观测表明，这不仅是可能的，而且是极其迅速的，尽管不是以一种平稳连续的方式。 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，这些研究已经确定湍流为快速磁场重联提供了充分条件。 我也将研究在不存在湍流和等离子体效应的情况下，由非平凡拓扑结构驱动的重连可以很快发生的情况。 这项工作的另一个主要目的是了解大尺度上磁场的产生，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这样大规模的场，而且在这两种情况下，它们的演化似乎都相对较快--恒星需要几年，星系可能需要10亿年。 磁螺旋度守恒，即磁场的“扭曲度”，已经被证明可以有力地限制这种场的增长。 我将使用分析工作和模拟来展示差异旋转系统（如恒星和星系）中的湍流如何自然地导致绕过这些约束的磁螺旋度的内部流动。