Magnetism Molds the Universe

In diffuse, weakly ionized matter, which forms a major part of the universe, the motion of each individual charged particle is strongly controlled by the magnetic field. For example, a hydrogen ion in the solar wind with a thermal velocity of 20 kilometers per second in the interplanetary magnetic field of 5 nanoteslas [the magnetic field at Earth's surface is about 50,000 nanoteslas] experiences a magnetic force about 107 times stronger than the gravitational force from the Sun.

Within the framework set by the larger magnetic field, the charged particles of a plasma engage in their own complex electromagnetic interactions, with the net effect being the widely observed filamentary currents. Since the charged particle movements sweep up the neutral matter with them, the claim that magnetic fields sculpt the universe thus follows quite readily. Applying patterns already discerned in plasmas from the laboratory out to interplanetary space, we may reasonably infer a hierarchy of magnetic fields in the universe. These would be associate with a hierarchy of electric currents, such that currents at one level would induce the magnetic fields that guide smaller-scale current filaments.

Large-scale magnetic fields have already been discovered in distant cosmic objects. At the center of the Milky Way, magnetic field on the order of 100s of nanoteslas in strength and stretching 195 light-years have been detected. Similar strengths are inferred from polarization measurements of radiation recorded for double radio galaxies.

At cosmic scales, scientists have now confirmed the existence of such filaments in the space between stars, as is discussed in a following sidebar. Clearly such filaments are the source

of magnetic fields that must impact plasma dynamics at smaller scales.

In the solar "neighborhood"—the region of space extending out from the Sun a distance of about 6,500 light-years, or one-eighth the diameter of the Milky Way—the strength of the magnetic field has been determined to be about 0.4 nanotesla, which is sufficient to account for the minimum radio emission between the galaxy's spiral arms.

 


That magnetic fields shape galaxies is supported by the close correspondence between observed magnetic fields (green arrows) and magnetic fields from supercomputer models (blue ribbons) of the interaction of two intergalactic-current filaments.

One of the most compelling pieces of evidence for the existence of supercluster-sized currents comes from the discovery of faint supercluster-scale radio emissions at 326 megahertz between the Coma galaxy cluster and the Abell 1367 cluster. Given that radiation of that frequency must be produced by free electrons moving at certain very high speeds, we can infer magnetic-field strengths of 0.03–0.06 nanotesla stretching for some 490 million light-years. This corresponds to a galactic current of nearly 1019 amperes.

—A. P. and G. C. S.

 

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