Modeling astrophysical outflows via the unified Dynamo-Reverse Dynamo mechanism

TL;DR

This paper introduces a unified Dynamo-Reverse Dynamo mechanism based on Hall magnetohydrodynamics, explaining how large-scale astrophysical outflows and magnetic fields are generated from microscopic reservoirs, with implications for observed outflows.

Contribution

It develops a new theoretical framework unifying magnetic field and outflow generation, highlighting the role of the ion skin depth and the Alfvén Mach number in astrophysical contexts.

Findings

The Dy-RDy mechanism derives from Hall MHD, linking microscopic and macroscopic scales.

The Alfvén Mach number relates to the ambient microscopic scale length.

Efficient reverse dynamo may explain high Mach number astrophysical outflows.

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 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 Alfv\'en Mach number $\mathcal{M}_{A}$, defining the relative "abundance" of the flow field to the magnetic field is shown to be tied to a microscopic scale length 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 $\mathcal{M}_{A} \gg 1$.