Theoretical Plasma Astrophysics

Astrophysical systems – planets, stars, galaxies, galaxy clusters, and possibly the intergalactic medium at large – carry magnetic fields.  Magnetic fields play an important role in energy and momentum transport, can  rapidly release energy in flares, and  are required to accelerate the relativistic particles known as cosmic rays. Despite many decades of progress in cosmology, we still do not know when or how magnetic fields originated in the Universe.  Plasma astrophysics is the study of how astrophysical systems interact with electromagnetic fields, and how the fields originated.

UW-Madison is an exceptionally good place to study plasma astrophysics. In addition to the astronomy and astrophysics program, there is an excellent plasma physics program, and we are the lead institution for the Center for Magnetic Self-Organization, which studies basic plasma processes and their application to astrophysics. The IceCube project is also a source of inspiration.

Run by Ellen Zweibel and Jay Gallagher.

Cosmic Rays and Galactic Winds

The accompanying figure shows the inner Milky Way in soft x-rays. It is indicative of  hot gas, probably heated by supernovae – the star formation rate, and hence the supernova rate, are very high in the inner Galaxy. Supernovae are known to  accelerate cosmic rays, so cosmic rays also must be present. We made a model of a wind from the inner galaxy which is driven by gas pressure and cosmic ray pressure, in roughly equal amounts (Everett et al. 2008, 2010) which is able to fit the observations.

How does cosmic ray pressure actually drive thermal gas? This is a bit subtle. Cosmic rays, after all, are relativistic particles. They should just be able to stream out of the galaxy at the speed of light. However, it turns out that  a large flux of cosmic rays streaming through a magnetized plasma leads to the growth of magnetic fluctuations. These fluctuations extract momentum and energy from the cosmic rays, and transfer it to the gas. The end result is that the cosmic rays and thermal gas are strongly coupled together. This works very nicely in hot, fully ionized gas. It doesn’t work well at all, however, in cool interstellar clouds (Everett & Zweibel 2011), because the fluctuations are rapidly dissipated in cold gas. This means that the cooler gas seen in outflows from other galaxies must be driven by another mechanism, or must have formed once the wind had already been accelerated.

We are applying similar ideas to other galaxies, where conditions can be quite different. The starburst region of M82, where the cosmic ray density, gas density, and magnetic fieldstrength are all very high, is a good example. Here, the wind may be an evaporative flow.

Astrophysical Dynamos

A dynamo is a mechanism for converting flow energy to magnetic energy. The magnetic fields of most stars and galaxies are probably sustained by dynamos. The flow energy has an ordered component – the large scale differential rotation, and a turbulent component. Both are important.

The solar dynamo, and some stellar dynamos, are cyclic in time, with the cycle period of the solar dynamo being 22 years, and other cycles being observed on many different stars.  One way to study stellar dynamos is through numerical simulations.  The accompanying figure shows a “magnetic wreath” prodiced by one of Ben Brown’s dynamo simulations. The magnetic field is stretched by rotation into a ring in a plane perpendicular to the rotation axis. Eventually we would like to understand the relationship between stellar rotation, luminosity, and the length and vigor of its magnetic cycles.

Dynamos can also be studied in the laboratory. There are two dynamo experiments at UW-Madison. The Madison Dynamo Experiment is a ball of liquid sodium that can be mechanically stirred. The Madison Plasma Dynamo Experiment will be a ball of plasma that can be used to to study a host of dynamo problems, inlcuding some related to the growth of magnetic fields in the early universe.