Magnetic fields in intergalactic space—revealed through synchrotron radiation from cosmic relativistic electrons—are stronger than one might expect. At up to tens of microgauss, they carry a significant fraction of the energy in the rarefied plasmas they pervade. (See the article by Philipp Kronberg, Physics Today, December 2002, page 40.)
Although the origin of the fields is still largely a mystery, theorists have some ideas about how they might arise. One favorite model begins with the Biermann battery mechanism: Plasma temperature and pressure gradients pointing in different directions generate a net electric current, which creates minute magnetic fields of about 10−21 G. Those seed fields are then amplified by turbulence: When the plasma is churned—in a collision between galaxy clusters, for example—the field lines remain pinned to their plasma, so they stretch and twist. As individual field lines are elongated, their overall spatial density—and thus the field—becomes greater....
