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An Aside on MHD

© Adam Hughes

This article was initially intended to be all about the simulation of Active Galactic Nuclei (AGN) jets as our first foray into all things computational and astrophysical. We'll still get into this topic a little bit here, and probably more later on, but a funny thing happened on the way to the black hole. A funny thing called magnetohydrodynamics, or MHD.

You see, AGN jets are essentially the spewing back of matter FROM a black hole, and the mechanism governing their formation has apparently been the topic of debate and intense investigation. Since we don't have any black holes here on Earth to study, scientists have had to rely on computer models to try to get a grasp of this phenomena. To date, the model that has shown the most hope is one which treats the jet as being a magnetically driven flow of particles, developed by Shibata and co-workers at the National Astronomical Observatory of Japan (to see a description of their work, visit http://spot.mtk.nao.ac.jp/katosi/cdrom/ ). As it turns out, the principles of magnetohydrodynamics can be used to describe such a system.

Magnetohydrodynamics can be thought of as an extension of traditional fluid dynamics, which is the theory of motion in fluids (gas, liquid, plasma) and by fluids. Fluid dynamics has been used successfully in airplane design, water body control, and climate prediction, to name just a few focus areas. Magnetohydrodynamics introduces into this scenario an electrical current (in the fluid) interacting with some external magnetic field. This theory has been put to use in areas as disparate as plasma confinement, nuclear reactor cooling, and even water softeners!

Given the truly impressive spectrum of scientific and practical arenas where MHD has the potential to have a large impact, it is not too surprising that flow problems involving a current and a magnetic field have been vigorously pursued. Because of the complexity of the problem, however, only a few simple cases have been solved rigorously. This is often the case in the theoretical branches of science, and it is probably unreasonable to expect every problem to have a definite numerical solution. In spite of that fact, research continues to find ways around the traditional sticking points. In the coming weeks, we'll look at MHD simulation in a little more detail before hopping back in our space ships and exploring the far reaches of the Universe. In the meantime, you can learn more about MHD at http://www.auburn.edu/~ajm/mhd/mhd.html

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