UC Santa Barbara

In this dissertation we focus on the accretion disks which surround accreting white dwarfs as they are some of the most abundant and well observed accretion disk systems. In many of these systems (e.g. dwarf novae), the accretion disk switches between a low luminosity state (quiescence) and a high luminosity state (outburst). These outbursts enable observers to place numeric constraints on the strength of turbulence (i.e. the α parameter) in these accretion disks. This dissertation focuses on results of local (stratified shearing-box) computer simulations of white dwarf accretion disks, and uses these results to gain a better theoretical understanding of these disks. As expected we find that the magnetorotational instability (MRI) is the predominant source of turbulence in these systems. However, we also find that hydrodynamic convection plays a key role as well. During the high luminosity state the disk becomes convectively unstable and the resulting convection enhances the MRI by seeding it with vertical magnetic field. This provides the first robust theoretical mechanism for enhancing turbulence only in outburst; a result required by observations. This convection also prevents the magnetic dynamo in our simulations from exhibiting the typical behavior of magnetic field reversals propagating vertically throughout the simulation. We also examine how the convection in our simulation changes the prior theoretical understanding of these disks. Specifically, we examine how these disks change luminosity over time by generating synthetic lightcurves using a modified disk instability model. These models can successfully reproduce observed outburst and quiescence durations, as well as outburst amplitudes. However, these lightcurves exhibit reflares in the decay from outburst, which are not generally observed in dwarf novae. Although, we highlight the problematic aspects of the quiescence physics in the disk instability model and MRI simulations that are responsible for this behavior.