Magnetic Field Evolution in Astrophysics

点击翻译按钮获取中文摘要

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.