Understanding the Large Scale Poloidal Magnetic Field Dynamo in Black Hole Accretion.

The production of relativistic jets by accreting black holes requires the presence of strong poloidal magnetic fields near the event horizon. However, most known physical processes are thought to strengthen magnetic fields in the toroidal directions. Recently, numerical GRMHD simulations were used to show that accretion disks with a purely toroidal magnetic field can create poloidal magnetic fields strong enough to launch jets via a large-scale dynamo (Liska et al. 2020). Prior to this analysis, it seemed likely that this dynamo functioned similarly to the alpha-omega dynamo, which generates poloidal fields in the Sun. In this presentation, I will summarize work that I conducted to determine the dynamo's mechanism. The dynamo creates four loops of alternating polarity. To determine if the alpha-process could account for these changes in polarity, I analyzed how the expansion, compression, and velocity of the fluid vary throughout the disk, and how they relate to each other. I found that changes in the expansion / compression and direction of motion of the fluid across different regions of the disk correspond to changes in loop polarity. However, these changes appear inconsistent with the alpha-process. Nonetheless, they provide strong clues to the dynamo’s actual mechanism.