MHD Turbulence in Accretion Disk Boundary Layers

TL;DR

This paper investigates the behavior of magnetohydrodynamic turbulence in accretion disk boundary layers, revealing that magnetic energy can be amplified but angular momentum transport remains inefficient, challenging standard viscous models.

Contribution

It provides new insights into the structure of MHD turbulence in accretion boundary layers, highlighting discrepancies with traditional shear viscosity assumptions.

Findings

Magnetic energy density can be significantly amplified in the boundary layer.

Angular momentum transport is inefficient despite magnetic energy amplification.

Results align with numerical simulations showing limited angular momentum transfer.

Abstract

The physical modeling of the accretion disk boundary layer, the region where the disk meets the surface of the accreting star, usually relies on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. The standard model for turbulent shear viscosity, widely adopted in astrophysics, satisfies this assumption by construction. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability is inefficient in this inner disk region. I will discuss the results of a recent study on the generation of hydromagnetic stresses and energy density in the boundary layer around a weakly magnetized star. Our findings suggest that although magnetic energy density can be significantly amplified in this region, angular momentum transport is rather inefficient. This seems consistent with the results obtained in numerical simulations and suggests that the detailed structure of turbulent MHD boundary layers could differ appreciably from those derived within the standard framework of turbulent shear viscosity.