Space is threaded with magnetic fields. They exist around galaxies, stars, and even in nearly empty voids — the vast empty spaces between galaxy clusters. It's usually thought that fields are amplified from weak 'seed' fields by turbulence, much like a dynamo. But the origin of the seed fields themselves has remained a puzzle.
New work proposes an unexpected mechanism. In tenuous cosmic plasma (hot gas of charged particles) where collisions are rare, electrons can move at different speeds in neighboring regions. Friction arises between flow layers — scientists call this phase mixing. It slows electrons, but unevenly: fast ones decelerate more quickly, slow ones more slowly. This difference in braking generates an electric current. And a current always creates a magnetic field. Thus, in completely uncharged plasma, a magnetic field generator spontaneously switches on.
The authors tested the idea in three ways: analytical calculations, computer simulations (so-called kinetic simulations that track each particle), and estimates for turbulent plasma. The result surprised them: the field is coherent on scales far exceeding the size of the particles themselves. Its strength is comparable to, and sometimes exceeds, that from the classic Biermann battery (another well-known generation mechanism).
This discovery changes our view of cosmic magnetism. If fields can be born literally from nothing, in any clump of moving gas, then the universe should have been riddled with weak magnetic fields from the very beginning. Perhaps this is exactly how the seed fields that galactic dynamos later amplified arose. Now astrophysicists must figure out how important this process was in the early universe and how it influenced the formation of the first stars and galaxies.
Earth's magnetic field is also created by the motion of charged fluid — molten iron in the core. But there the dynamo mechanism operates, not phase mixing: particles collide frequently, and currents flow quite differently.