The influence of accretion disk thickness on the large-scale magnetic dynamo

TL;DR

This study investigates how the thickness of accretion disks influences the behavior of large-scale magnetic dynamos, revealing that thicker disks suppress organized dynamo activity and alter variability characteristics.

Contribution

The paper demonstrates that increasing the disk scale height ratio disrupts the large-scale dynamo organization and modifies the magnetic and photometric variability patterns in accretion disks.

Findings

Large-scale dynamo fails to organize in disks with h/r > 0.2

Thinner disks exhibit characteristic butterfly magnetic field patterns

Disks with unordered dynamo show larger stochastic photometric fluctuations

Abstract

The evolution of the magnetic field from the large-scale dynamo is considered a central feature of the accretion disk around a black hole. The resulting low-frequency oscillations introduced from the growth and decay of the field strength, along with the change in field orientation, play an integral role in the accretion disk behavior. Despite the importance of this process and how commonly it is invoked to explain variable features, it still remains poorly understood. We present a study of the dynamo using a suite of four global, high-resolution, MHD accretion disk simulations. We systematically vary the scale height ratio and find the large-scale dynamo fails to organize above a scale height ratio of $h/r \gtrsim0.2$. Using spacetime diagrams of the azimuthal magnetic field, we show the large-scale dynamo is well-ordered in the thinner accretion disk models, but fails to develop the characteristic "butterfly" pattern when the scale height ratio is increased, a feature which is also reflected in the power spectra. Additionally, we calculate the dynamo $\alpha$-parameter and generate synthetic light curves. Using an emission proxy, we find the disks have markedly different characters as stochastic photometric fluctuations have a larger amplitude when the dynamo is unordered.