George Field is a theoretical astrophysicist at the Harvard-Smithsonian Center for Astrophysics, where he has been since 1973. The founding director of the Center in 1973, he stepped down in 1982 to pursue teaching and research.
In recent years, he has become increasingly interested in the physics of turbulence in electrically conducting fluids in which a magnetic field is embedded. Such fluids are found everywhere in the universe, including the sun and stars, and the interstellar medium between the stars. Turbulent motion is directly observed in the stream of gas coming from the sun known as the solar wind, and is present in the interstellar medium that permeates our Galaxy and other galaxies. How did the magnetic field of the Galaxy originate? This question has not been answered for sure, but the mechanism probably requires the presence of something called helicity. Helicity is a measure of the linkage between curves or lines in space. For example, kinetic helicity measures the linkage of vortex lines, and current helicity measures the linkage of lines of electric current. Neither of these is conserved in flows in which the viscosity and electrical resistivity is very small.
However, magnetic helicity, which measures the linkage of magnetic lines of force, is conserved, and that seems to be intimately related to the origin of galactic magnetic fields.
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| The twisting ropes are similar to the twist in magnetic fields. |
Field is attempting to understand this in as general a way as possible so as to throw light on the magnetic fields that are so ubiquitous in astronomy.
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| The interstellar polarization of starlight plotted in galactic coordinates by Mathewson and Ford (1970). The length and position angle of each line indicate the degree of polarization, P, and the plane of vibration of the electric vector, respectively. |
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| The orientation of the large-scale magnetic fields in M51 from Neiminger (1992) using the polarization of synchrotron radiation. |
As can be seen in the figures, a large-scale magnetic field permeates the interstellar medium of our Galaxy and of the galaxy Messier 51. As far as we can tell, the same is true of all galaxies like our own. This fact presents a puzzle. Interstellar gas contains enough ion-electron pairs that it can easily carry electric current. As in a coil of wire, this current creates a magnetic field. Any resistance present causes the current, hence the magnetic field, to decay. But by Faraday's law, the decaying magnetic field induces a voltage that compensates for the resistance, thus slowing the decay of the current and the field.
For an electric circuit in the laboratory, this postpones the decay of the field for a few milliseconds. But because the induction effect is proportional to the size of the system, and the resistance is inversely proportional to the size, the time constant for the decay of the magnetic field of a galaxy is vastly greater, some 10 billion times the age of the universe. At first sight this seems like a good thing, because we observe that a field is present today. On the other hand, what accounts for the presence of a magnetic field in the first place? Just as the present field can change only very slowly from its present value, the field present when the galaxy was formed would have changed very slowly from its initial value. Either the galaxies were born with their present magnetic fields, or else something is missing from the argument.
The whole argument is changed if turbulence is taken into account. Turbulence breaks large scale fields into small scale fields, which can be dissipated, so an initial field would last a much shorter time - only a hundredth of the age of the Galaxy. The question is thus turned around, and instead becomes what regenerates large-scale fields in spite of turbulent dissipation? Ever since the pioneering work of Eugene Parker in 1955 and 1971, most astronomers have assumed that the answer is an "alpha-omega dynamo", in which helical turbulence plays a key role. Such turbulence, in which the turbulent velocity is correlated with the turbulent vorticity, can amplify a large scale field by the so called "alpha effect".
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| Parker's "alpha effect" due to helical turbulence. |
Recently, however, controversy has arisen over the value of the field strength at which the large scale field saturates. A natural estimate is that at which the magnetic energy is in equipartition with the turbulence, and that is consistent with the observations sketched above. But some simulations seem to indicate that the saturation field is vastly smaller. In the first paper, we show that in a simple case, the amplification saturates at the field strength one expects naively.
In the second paper we show that a recent proposal for creating a "seed field" in the early universe, for the turbulence to amplify, produces a field on a vastly larger scale than one might expect, because the creation mechanism creates a field with magnetic helicity. Since the alpha effect depends upon kinetic helicity, it appears that helicity is involved in both the creation and amplification of galactic magnetic fields.
Current Thinking
The scene is getting exciting. In the article I cited with Sean Carroll, I predicted that the Joyce- Shaposhnikov mechanism could create a helical magnetic field in the early universe, which would be available for incorporation into the intracluster gas in clusters. We found an upper limit on the scale of the field of 20 Kpc, corresponding to 1 Kpc in the gas before clusters began to collapse at a redshift of 15, and an upper limit of 4 x 10-10 Gauss on the magnetic field strength in the IGM today, corresponding to 8 x 10-8 at the time of cluster collapse. In a recent preprint, Dolag, Bartelmann, and Lesch worked out what the precollapse field would have to be in order to explain observations of the Coma cluster, and found 10-9 Gauss, smaller than our upper limit. The observed scale is larger than we would predict, however, but not hugely so. Recall that the J-S mechanism produces a helical field, a fact that is crucial in attaining astronomically interesting scales today. There is evidence from other clusters that the field is helical in shape. Could it be that cluster magnetic fields were created in the early universe? More work is needed. The prospect is exciting because it constrains parameters of the early universe - and there are few such constraints.
Look at a beautiful picture from the Hubble Space Telescope.