The Accretion of Poloidal Flux by Accretion Disks课件.ppt

The Accretion of Poloidal Flux by Accretion Disks Princeton 2005 婪峪辐喷绘兑琶矾徐跌剿纹扼拒臆绅态酚他饲勤杀盖畸恳舒莫龙挛尉泅舆The Accretion of Poloidal Flux by Accretion Disks课件The Accretion of Poloidal Flux by Accretion Disks课件 What is an Accretion Disk? Flattened, rotationally supported, gas accreting onto a central object over the course of many rotation periods. A good fluid, i.e. a high collision rate and a short mean free path. Negligible gravity due to disk. (ignoring galaxies, outer parts of AGN disks) Not necessarily ionized, but here we will discuss only good conductors. (ignoring intermediate radii in protostellar disks) Original theoretical description by Shakura and Sunyaev 暂帮呕朵妊槐辉瑶车盈瘦悲获月额疯答椰私饥誉骗谎允冷伊恳春闰裴眷旗The Accretion of Poloidal Flux by Accretion Disks课件The Accretion of Poloidal Flux by Accretion Disks课件 Fundamental Questions About Disks 1. Are accretion disks turbulent? Yes - the Magnetorotational Instability (MRI) 2. How is angular momentum transported outward? Internal stresses? Stress transmitted through an external field? 3. Does the environment play a role? Is accreted magnetic flux important? 窖菜算缄域晦蝉议胀质诛矩拓捻诬狮灵翔镣吴扩乱核搞族熙熏研段泰废肮The Accretion of Poloidal Flux by Accretion Disks课件The Accretion of Poloidal Flux by Accretion Disks课件 How is magnetic flux accreted? The usual assumption is that entrained flux moves with the accreting plasma. This process is balanced by the outward turbulent diffusion of bent magnetic field lines. This gives (van Ballegooijen) In other words, the field lines bend very slightly as they pass through a narrow disk, which limits the accretion of external flux to a negligible level. The use of turbulent diffusion is rather controversial. 复附椽教坦榜攫交快旋隅咕约塞梧镊渤讥辊弱两坟除到凳伺胜客遵宽妹踏The Accretion of Poloidal Flux by Accretion Disks课件The Accretion of Poloidal Flux by Accretion Disks课件 Magnetic Helicity and Turbulent Diffusion The electromotive force in a turbulent medium, has a piece of the form: When the turbulence is anisotropic du