The effects of vertical outflows on disk dynamos

Abstract

Abstract.We consider the effect of vertical outflows on the mean-field dynamo in a thin disk. These outflows could be due to winds or magnetic buoyancy. We analyse both two-dimensional finite-difference numerical solutions of the axisymmetric dynamo equations and a free-decay mode expansion using the thin-disk approximation. Contrary to expectations, a vertical velocity can enhance dynamo action, provided the velocity is not too strong. In the nonlinear regime this can lead to super-exponential growth of the magnetic field.

Structured outflow from a dynamo active accretion disc

“…In other words, nonlinear states of disc dynamos are almost insensitive to the detailed properties of α (e.g., Beck et al 1996;Ruzmaikin et al 1988). A discussion of disc dynamos with outflows, motivated by the present model, can be found in Bardou et al (2001). It is shown that the value of magnetic diffusivity in the corona does not affect the dynamo solutions strongly.…”

Outflows and accretion in a star-disc system with stellar magnetosphere and disc dynamo

“…The resulting magnetic field symmetry is roughly dipolar. This symmetry is seen both in threedimensional simulations of accretion disc dynamos driven by turbulence from the magneto-rotational instability and in solutions of the αΩ dynamo problem with the α effect negative in the upper disc half, provided that the accretion disc is embedded in a conducting corona (e.g., Brandenburg et al 1990;Brandenburg 1998;von Rekowski et al 2000;Bardou et al 2001). Further, we include α quenching which leads to the disc dynamo saturating at a level close to equipartition between magnetic and thermal energies.…”

Outflows from Dynamo-Active Protostellar Accretion Discs

“…The same is seen both in three-dimensional simulations and in solutions of the α dynamo problem; see Brandenburg (1998). Bardou et al (2001) found that vertical outflows do not change the symmetry or temporal behaviour of the dynamo-generated magnetic field. However, especially in the linear case for negative α in the upper disc half and finite-conductivity in the halo, an outflow tends to switch the disc dynamo off.…”

“…However, the dynamo still seems to be unable to produce a large scale poloidal field (which seems to be required for driving systematic outflows), and thermodynamics is not included to allow the disc to cool sufficiently to become geometrically thin. Other groups have focused on the dynamo aspect and have used thin disc theory as input to their models (Reyes-Ruiz and Stepinski, 1995) or have studied the effect of vertical outflows on the disc dynamo (Bardou et al, 2001).…”

“…• at the disc surface, which is favourable for magneto-centrifugal acceleration (Blandford and Payne, 1982). However, the Alfvén surface is so close to the disc surface at the outer parts of the disc that acceleration there is mainly due to the gas-pressure force.…”