Centre for Information Science Research Machine CM, VP

Co-Investigators Dayal T Wickramasinghe, Robert L Dewar and Geoffrey V Bicknell

Department of Mathematics, Research School of Physical Sciences and Engineering and

Mount Stromlo and Siding Spring Observatories

MHD Phenomena in Accretion Systems

Magnetohydrodynamics plays a decisive role in many astrophysical phenomena. Solar activity periods, radio-storms in the Earth magnetotail, pulsars, and possibly quasi-periodic outbursts in dwarf-novae are all driven by magnetohydrodynamics. Even weak magnetic fields when dragged by mechanical motions of plasma can become entangled and amplified. This is especially true of accretion systems in which the entanglement is additionally enhanced by the geometry of the flow. Local knots of magnetic field can relax explosively in what is known as magnetic reconnection. Magnetohydrodynamic turbulence is another phenomenon which may be responsible for very high observed viscosities in accretion systems. Geertsema and Achtenberg used the cascade model to simulate magnetic turbulence in a differentially rotating accretion disk and demonstrated that a very high effective viscosity could be obtained in this way. This result cannot be reproduced without the presence of magnetic fields. Within this project we attempt to study a variety of astrophysical systems characterised by the presence of a central compact massive body and magnetic fields both embedded in the plasma as well as generated by external sources.

What are the basic questions addressed?

What is the basic topology of flows, currents, and magnetic fields that develops during the accretion event for various initial and boundary conditions?

What are the results to date and the future of the work?

We have carried out two large supercomputer simulations. The first one was concerned with the evolution of a young accretion disk in a compact binary system. That simulation demonstrated that a Karman vortex path forms downstream from the point of impact. The second simulation was concerned with the gravitational collapse of a magnetised vortex. This simulation has been related to the formation and properties of the 2pc circumnuclear ring in the Galactic Centre.

What computational techniques are used and why is a supercomputer required?

Smoothed Particle Magnetohydrodynamics is used to carry out the simulations. All simulations are 3 dimensional. This implies that very large data sets corresponding to at least half a million of Lagrangian nodes must be handled. At present that type of work can be carried out on a supercomputer only.

Publications

3d structure of truncated accretion disks in close binaries , Z Meglicki , D Wickramasinghe,
G V Bicknell, Monthly Notices of the Royal Astronomical Society, 264, 691-704, (1993).

Verification and accuracy of smoothed particle magnetohydrodynamics, Z Meglicki, Computer Physics Communications, in press.

Gravitational collapse of a magnetised vortex -- application to the galactic centre, Z Meglicki, D Wickramasinghe, R L Dewar, Monthly Notices of the Royal Astronomical Society (1994), submitted.