Trapped inertial waves
So-called `High-frequency quasi-periodic oscillations' (HFQPOs) observed in the emission from black hole binary systems provide a potential probe into the structure of strongly curved space-time, but are still poorly understood. One explanation offered is that the oscillations are cause by diskoseismic trapped inertial waves (r-modes) confined by relativistic effects to a `self-trapping region.' In the un-magnetized, hydrodynamic theory, r-modes provide an attractive model: their frequencies can be directly related to the mass and spin of the central black hole, and they are readily excited by warping or eccentric deformations in the disk.
An illustrative contour-plot showing the pressure perturbation (in radius and height above/below the disk) associated with a trapped inertial wave in the inner regions of a black hole accretion disk (with spin parameter a=0.5).
Trapped inertial waves' natures are changed by magnetic fields, however, which provide restoring forces through magnetic tension and pressure. In my PhD I have worked to precisely quantify the effects of magnetic fields with different geometries on r-modes through a semi-analytical approach, while also running numerical simulations to investigate the resonant excitation of r-modes through coupling with eccentric streamlines in the flow.
An exagerated illustration of a magnetohydrodynamic r-mode's distortion of a magnetic field threading the disk purely perpendicularly.
The magnetorotational instability (MRI) provides a widely accepted explanation for turbulent accretion in astrophysical disks. However, although studied extensively in local simulations, numerical capabilities have only recently allowed for the dynamics of MRI turbulence to be explored on a global scale. I am interested in how the MRI might drive, damp or otherwise interact with other waves and instabilities theorized to occur on large scales in astrophysical accretion flows.
A slice showing the radial magnetic field perturbation associated with a large-scale MRI mode in the linear stage of growth.
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- Dewberry J, Latter H, Ogilvie G, "Quasi-periodic oscillations and the global modes of relativistic, MHD accretion discs," MNRAS, Vol. 476:4085–4103, 2018
- Elmegreen B, Elmegreen D, Almeida J, Muñoz-Tuñón C, Dewberry J, Putko J, Teich Y, Popinchalk M. "Massive clumps in local galaxies: Comparisons with high-redshift clumps." ApJ, 774:86, 2013
- Elmegreen D, Elmegreen B, Almeida J, Muñoz-Tuñón C, Putko J, Dewberry J. "Local tadpole galaxies." ApJ, 750:95, 2012