Radially Extended, Stratified, Local Models of Isothermal Disks

TL;DR

This paper introduces radially extended, stratified local models of isothermal accretion disks to explore mesoscale structures, magnetic field configurations, and oscillatory behaviors, revealing key insights into turbulence, magnetic correlations, and dynamo activity.

Contribution

It presents a novel modeling approach with specific radial and azimuthal extents, enabling detailed analysis of mesoscale phenomena in stratified thin disks.

Findings

Weakly magnetized midplane with strong corona

Coronal magnetic fields correlated on ~10 H scales

Quasi-periodic oscillations linked to alpha-dynamo

Abstract

We consider local, stratified, numerical models of isothermal accretion disks. The novel feature of our treatment is that radial extent L_x and azimuthal extent L_y satisfy H << L_x, L_y << R, where H is the scale height and R is the local radius. This enables us to probe mesoscale structure in stratified thin disks. We evolve the model at several resolutions, sizes, and initial magnetic field strengths. Consistent with earlier work, we find that the saturated, turbulent state consists of a weakly magnetized disk midplane coupled to a strongly magnetized corona, with a transition at |z| ~ 2H. The saturated \alpha ~ 0.01 - 0.02. A two-point correlation function analysis reveals that the central 4H of the disk is dominated by small scale turbulence that is statistically similar to unstratified disk models, while the coronal magnetic fields are correlated on scales ~ 10 H. Nevertheless angular momentum transport through the corona is small. A study of magnetic field loops in the corona reveals few open field lines and predominantly toroidal loops with a characteristic distance between footpoints that is ~ H. Finally we find quasi-periodic oscillations with characteristic timescale ~ 30 \Omega^{-1} in the magnetic field energy density. These oscillations are correlated with oscillations in the mean azimuthal field; we present a phenomenological, alpha-dynamo model that captures most aspects of the oscillations.