Physics Highlights, 2008
Discovery of an electron diffusion region in a reconnection layer of a laboratory plasma
February 21, 2008
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An electron diffusion region has been identified for the first time in the reconnection layer of a laboratory plasma. Magnetic reconnection can alter the global structure of magnetic field. However, the rate of reconnection is controlled in part by dynamics in a reconnection region (sometimes called the diffusion region) about which magnetic field lines tear and reconnect. Recent 2D numerical simulations predict a two-scale diffusion layer in which an electron diffusion layer resides inside of the larger ion diffusion layer of the width of the ion skin depth. In the Magnetic Reconnection Experiment (MRX) at the Princeton Plasma Physic Laboratory, the electron diffusion region is verified and found that demagnetized electrons are accelerated in the outflow direction. The measured width of the electron diffusion region scales with the electron skin depth (˜8c/ωρi, where ωρi is the electron plasma frequency) and the electron outflow scales with the electron Alfv’en velocity ( 0.11VA). Properties of both the electron and ion flow structures agree with simulation.

Transport and electric field structures from magnetic chaos arising during reconnection
February 21, 2008
Measurement of the charge flux (or current) vs radius across a toroidal plasma, arising from the chaotic magnetic field that accompanies magnetic reconnection. (Courtesy: W. Ding, UCLA)
In some laboratory and astrophysical situations, magnetic field lines can wander chaotically in space. In the laboratory, this occurs during magnetic reconnection. Since particles follow field lines, the particle trajectories of motion can also become chaotic. In the MST experiment at the University of Wisconsin, the particle transport arising from magnetic chaos has been measured. The magnetic field and the particle motion (that appears as an electrical current) are measured by injecting lasers through the plasma. The magnetic field and current are then deduced by the effect of the plasma on the lasers (an effect known as Faraday rotation). The transport of the negative electrons and positive ions are observed to be unequal, causing an electric field to arise around the location where reconnection occurs. An implication derived from the electric field is that a strongly sheared plasma flow should arise at the reconnection location, thereby introducing the notion that chaotic magnetic fields can be a source of flow generation. Future experiments will search for such flows.

Galactic Winds Driven by Cosmic Rays
February 21, 2008
Soft x-ray emission from the inner Milky Way galaxy, showing the presence of hot (several million degree) gas. Figure from J. Everett, E. Zweibel, R. Benjamin, D. McCammon, L. Rocks, J. Gallagher, ''The Milky Way’s Kiloparsec-Scale Wind,'' Astrophysical Journal, V. 674, 258 (2008).
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The inner parts of our galaxy may be a dramatic example showing how small scale fluctuations can act in concert to drive large scale flows. The figure shows x-ray emission from the inner galaxy, emitted by gas at several million degrees. The gas was probably heated over millions of years by multiple supernova explosions, and in previous models it has been assumed to be at rest, supported against the galactic gravitational field by its own pressure. However, supernovae also create the relativistic nuclei known as cosmic rays. As the cosmic rays stream away from the supernovae, they excite short wavelength magnetic fluctuations, which transfer momentum to the gas. We showed that this process can impart enough momentum to actually drive the gas out of the galaxy. This galactic wind carries away about 2 solar masses of gas each year, making it important for galactic evolution. The wind model is a better fit to current observations than the static model, and we are in the process of developing additional observational tests based on the synchrotron and gamma radiation expected from cosmic rays in the wind. Cosmic ray driven winds of this type may also have been important in young galaxies, and for heating the intergalactic medium.

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Magnetic Field Generation by Plasma Turbulence in Astrophysical Disks
February 21, 2008
Supercomputer simulations of magneto-rotational turbulence in a cylindrical annulus. In this simulations the magnetic field was regenerated by the turbulent motions. The figure shows the azimuthal velocity fluctuations. (Couresy of F. Cattaneo, University of Chicago)
The process of accretion plays a fundamental role in astrophysics: it allows the formation of stars and planets and the release of gravitational energy to power some of the most energetic phenomena in the universe. The rate of accretion is controlled by the outward transport of angular momentum; without it the material would just orbit the central object and never accrete. Over a decade ago, it was realized that magnetic fields could lead to the development of turbulence in astrophysical discs, which in turn would enhance the transport of angular momentum and explain the observed accretion rates. One of the questions that arose concerned the origin of the magnetic fields: were they external to the disc, or could they be generated by dynamo processes by the very turbulence within the disc? The answer to this question has been the subject of considerable controversy.

CMSO researchers have addressed this issue by simulating the type of turbulence that is believed to occur in accretion discs and to show that if the magnetic diffusivity is sufficiently small—a condition that although common in astrophysical situations is extremely difficult to reproduce numerically---dynamo action ensues and the magnetic fields can indeed be generated by the turbulence. Furthermore, the simulations also verified that the resulting self-maintaining motions could efficiently transport angular momentum outwards. The more numerically demanding simulations were carried out on the Blue-Gene supercomputer at the IBM Watson Facility with up to 64,000 processors. They are, to date, the largest simulations of this type.