Physics Oral Defense: Dissipation of Turbulence in Black Hole Accretion Flows

Kaushik Satapathy, Physics Graduate Student


1 to 4 p.m., June 19, 2024

Abstract: The black hole images recently obtained with the Event Horizon Telescope (EHT) have provided a unique opportunity to study plasma astrophysics in these environments. In the first part of this dissertation, I develop methods to extract signatures of plasma processes using EHT observations, particularly by measuring the structural variability of the images. I find that current numerical models exhibit significantly higher variability than data. I interpret it as a consequence of simplistic parametric models of electron heating in the small dissipation scales that cannot be resolved in simulations. I build a physically consistent prescription for the observable electron temperatures by combining recent microphysical studies that examine the ion-to-electron heat partition resulting from collisionless damping of turbulence with a covariant analytic transport model of an accretion disk. I show that the ion-to-electron temperature ratio in the inner regions of accretion disks primarily depends on the plasma $\beta$ parameter and the relative composition of the slow- and the Alfv\'en wave cascades in the turbulence driving the flow at the large scales. I then study the mechanism of injection of turbulent energy into slow- and Alfvén- wave cascades in magnetized shear flows. I show that this ratio depends on the particular components of the Maxwell and Reynolds stress tensors that cause the transport of angular momentum, the shearing rate, and the orientation of the mean magnetic field with respect to the shear. I use numerical magnetohydrodynamic shearing-box simulations with background conditions relevant to black hole accretion disks to compute the magnitudes of the stress tensors for turbulence driven by the magneto-rotational instability and derive the injection power ratio between slow and Alfvén wave cascades. I use these results to formulate a local subgrid model for the ion-to-electron heating ratio that depends on the macroscopic characteristics of the accretion flow, paving the way for bridging the large scale-separation and developing physical and self-consistent models for the thermodynamics of accretion flows. 

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