I am interested in the turbulence that prevails in astrophysical disks and the instabilities that sustain it. Turbulence helps the disk material lose angular momentum so that it might accrete on to the central object: the great luminosity of many sources (AGN especially) are powered by the gravitational energy liberated by this process. Turbulence also influences the process of planet formation in protostellar disks. The magnetorotational instability (MRI) is the main cause of activity in well ionised disks, such as dwarf novae, X-ray binaries, and active galactic nucleii. The above figure shows the evolution of the horizontally average magnetic field in a space-time diagram of simulated magnetorotational turbulence; the y-axis is disk height and the x-axis is time. The `butterfly patterns' are signatures of a clear dynamo cycle.

I have been spending some time understanding the basic properties of this instability and the turbulent flows that it induces. Topics of interest comprise (a) how the MRI can sustain a (nonlinear) dynamo, (b) its dissipative properties and how energy can be released on small scales in energetic bursts (flares), (c) the emergence and destruction of large-scale structure (channels and zonal flows), (d) how effectively the MRI can transport large-scale magnetic flux, (d) its connection to winds and jet launching, and (e) the role of non-ideal MHD.

Another avenue I have pursued is the interaction between the MRI and other instabilities that can occur concurrently, such as thermal instability, convective instability, and gravitational instability (GI). The combination of the thermal instability and the MRI in particular, leads to cycles of repeated outbursts in dwarf novae on timescales of days and weeks that amateur astronomers can observe. The combination of MRI and GI, on the other hand, might lead to the rich and violent dynamics exhibited by certain protoplanetary disks, such as FU Ori or EX Lupi outbursts. These, on contrast, occur on much longer (and non-human) timescales (100-1000 years).

Potential PhD topics, all mostly numerical, include
*Global simulations of black hole accretion: how does the global dynamo operate? What causes the emergence and destruction of large-scale flows and fields?
*Magnetic flux transport in global disk models
*Highly magnetised, vigorously accreting MRI states: how prevalent and numerically robust are they?
*The stability of steady magnetic winds, in ideal and non-ideal MHD.

Recent papers

*Magnetorotational instability and dynamo action in gravitoturbulent astrophysical discs (link)
*MRI turbulence and thermal instability in accretion disks (link)
*Magnetorotationally driven wind cycles in local disc models (link)
*The stress-pressure relationship in simulations of MRI-induced turbulence (link)
*Local and global aspects of the linear MRI in accretion disks (link)
*Local outflows from turbulent accretion disks (link)