34 - Turbulent MHD jet collimation and thermal driving

Summary

Abstract

We have argued that MHD turbulence in an accretion disk naturally produces hoop-stresses, and that in a geometrically-thick flow these stresses could both drive and collimate an outflow. We based this argument on an analogy of turbulent MHD fluids to viscoelastic fluids, in which azimuthal shear flow creates hoop-stresses that cause a variety of flow phenomena, including the Weissenberg effect in which a fluid climbs a spinning rod.

One of the more important differences between the Weissenberg effect and astrophysical jets is the source of power. In our previous analysis, we only considered the power due to the spin-down torque on the central object, and thus found that we could only drive an outflow if the central object were maximally rotating. Here we take into account the energy that is liberated by the accreting matter, and describe a scenario in which this energy couples to the outflow to create a thermodynamic engine.

Introduction

We wish to discuss here in simple language some of our ideas regarding jet collimation and acceleration. In this paper, we will concentrate on the basic intuitive notions rather than the mathematics, which we have discussed in print elsewhere (see references below).

Review: turbulence models and jets

We have argued (Williams 2001; see also Ogilvie 2001) that the stress due to magnetohydrodynamic (MHD) turbulence in ionized accretion disks — such as, but not limited to, the turbulence driven by the magnetorotational instability (MRI) — behaves more like the stress in a viscoelastic fluid than the stress in a viscous fluid.