Modelling astrophysical outflows via the unified dynamo-reverse dynamo mechanism

Abstract

The unified dynamo-reverse dynamo (Dy-RDy) mechanism, capable of simultaneously generating large-scale outflows and magnetic fields from an ambient microscopic reservoir, is explored in a broad astrophysical context. The Dy-RDy mechanism is derived via the Hall magnetohydrodynamics, which unifies the evolution of magnetic field and fluid vorticity. It also introduces an intrinsic length-scale, the ion skin depth, allowing for the proper normalization and categorization of microscopic and macroscopic scales. The large-scale Alfvén Mach number MA, defining the relative 'abundance' of the flow field to the magnetic field is shown to be tied to a microscopic scalelength that reflects the characteristics of the ambient short-scale reservoir. The dynamo (Dy), preferentially producing the large-scale magnetic field, is the dominant mode when the ambient turbulence is mostly kinetic, while the outflow producing reverse dynamo (RDy) is the principal manifestation of a magnetically dominated turbulent reservoir. It is conjectured that an efficient RDy may be the source of many observed astrophysical outflows that have MA ≪ 1.