Advection of matter and B-fields in alpha-discs

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Abstract
We have carried out and analysed a set of axisymmetric magnetohydrodynamic (MHD) simulations of the evolution of a turbulent/diffusive accretion disc around an initially unmagnetized star. The disc is initially threaded by a weak magnetic field where the magnetic pressure is significantly less than the kinetic pressure in the disc. The viscosity and magnetic diffusivity are modelled by two ‘alpha’ parameters, while the coronal region above the disc is treated using ideal MHD. The initial magnetic field is taken to consist of three poloidal field loops threading the disc. The motivation for this study is to understand the advection of disc matter and magnetic field by the turbulent/diffusive disc. At early times ( ≲ 400 orbits of the inner disc), the innermost field loop twists and its field lines become open. The twisting of the opened field lines leads to the formation of both an inner collimated, magnetically dominated jet, and at larger distances from the axis a matter-dominated uncollimated wind. For later times (>1000), the strength of the magnetic field decreases owing to field reconnection and annihilation in the disc. For the early times, we have derived from the simulations both the matter accretion speed in the disc um and the accretion speed of the magnetic field uB which is determined by measuring the speed of the inward motion of the inner O-point of the magnetic field in the equatorial plane. We show that the derived um agrees approximately with the predictions of a model where the accretion speed is the sum of two terms, first due to the disc's viscosity (which gives a radial outflow of angular momentum in the disc), and a second due to the twisted magnetic field at the disc's surface (which gives a vertical outflow of angular momentum). At later times the magnetic contribution to um becomes small compared to the viscous contribution. For early times we find that um is larger than the magnetic field accretion speed uB by a factor of ∼2 for the case where both the alpha parameters are equal to 0.1.

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