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
*Dissipative structures in magnetorotational turbulence
(link)
*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)