A Magnetic α ω Dynamo in Active Galactic Nuclei Disks - NMT Physics

– 19 –and with an epicyclic frequency of Ω K . We performed simple ballistic calculations of trajectoriesof particles launched from a point at the mid-plane of the disk with initial velocities in differentdirections in the horizontal plane. We obtained that at the time of maximum height of the plume,≈ T K /4, the position angle of the major axis of the ellipse is approximately −π/4 from the outwardradial direction e r . At the time of the fall back to the disk plane at ≈ T K /2, the major axis of theellipse is close to the azimuthal direction. Such a distortion in the shape of an otherwise cylindricalplume will only slightly affect the rotation of the entrapped toroidal flux and hence will not alterthe dynamo action.Before calculating the size or radius, r p , we first verify the adiabatic approximation in thatthe diffusion of radiation is fractionally small compared to the hydrodynamic displacements. Inthis circumstance of a Shakura–Sunyaev disk, this will allow us to treat the star-disk collisions asstrong shocks within the disk matter. Subsequently we will consider the thinner, lower densityRossby disks (Li et al. 2001b) where radiation transport will dominate over shock hydrodynamics.However, for the purposes of the dynamo, the production of helicity from either plumes will besimilar.4.1. Radiation Diffusion in the Collision ShockDuring star-disk collisions the total energy taken from the star is ≈ ΣvK 2 πR2 ⊙ . This energyis distributed over a column of radial extent, ∆R rad , due to radiation transport. For an estimateof ∆R rad one can take the distance from the star track where the sideways diffusion of radiationbecomes comparable with the advection of the radiation by the displacement of the disk matterwith the star velocity v K (since the velocity of strong shock is of the order of v K ). This results in∆R rad = c/3κρv K, (18)where for ρ we consider the density ahead of the shock in the undisturbed disk matter to comparethe radiation flux with the transport of energy and momentum by the shock. We assume κ =0.4cm 2 g −1 , Thompson opacity. Then using ρ from expression (A6) at r ab ,∆R rad = 10 8 cm = 1.4 · 10 −3 R ⊙ . (19)Since ∆R rad ≪ R ⊙ , the radiation will remain local to the shocked fluid. The state conditions inthis shocked matter will depend upon the rapid thermalization between the matter and radiation.The number of photon scatterings, n hν within the time of traversal of the radiation front,∆R rad , becomes( )n hν = (∆R rad κρ) 2 c/3 2 ( ) r= = 50 . (20)v K r abTherefore the radiation will be fully absorbed and thermalized with the gas within ∆R rad . Sincethe gas pressure is radiation dominated for r < r ab and the shock has a high Mach number,