Whenever people look out into the sky, we can perceive magnetic fields embedded in all of the astrophysical objects they view. This is true not just in the vicinity of stars and planets, but also in the vast space between galaxies and galaxy clusters. These fields are weak—much weaker than a refrigerator magnet—but they are dynamically important in the sense that they have a considerable impact on the universe’s dynamics. The origin of these cosmic magnetic fields remains one of the most fundamental mysteries in cosmology, despite decades of intensive attention and inquiry.
Scientists had already discovered how turbulence, the churning motion intrinsic to all fluids, might magnify preexisting magnetic fields via the so-called dynamo process. However, this astonishing discovery just added to the enigma. Where did the “seed” magnetic field come from in the first place, since a chaotic dynamo could only enhance an existing field?
We wouldn’t have a comprehensive and self-consistent solution to the formation of astrophysical magnetic fields until we knew how the seed fields formed. New research by MIT graduate student Muni Zhou, her advisor Nuno Loureiro, an MIT professor of nuclear science and engineering, and colleagues at Princeton University and the University of Colorado at Boulder provides an answer that demonstrates the basic processes that generate a field from a completely unmagnetized state to the point where the dynamo mechanism takes over and amplifies the field to the magnitudes that we observe.
Magnetic fields are everywhere
Magnetic fields are found naturally everywhere in the cosmos. They were initially noticed on Earth thousands of years ago, as a result of their interaction with magnetic minerals such as lodestone, and were used for navigation long before people understood their nature or origin. The effects of magnetism on the sun’s spectrum of light were identified at the beginning of the twentieth century. Since then, more powerful telescopes scanning deep into space have discovered that the fields are everywhere.
While scientists have long known how to create and employ permanent magnets and electromagnets for a variety of applications, the natural sources of magnetic fields in the cosmos remained a mystery. Part of the answer has come from recent research, but many elements of the subject are still up for discussion.
The dynamo effect amplifies magnetic fields
Scientists began to explore this issue by looking at how electric and magnetic fields are generated in the laboratory. Electric fields are formed when conductors, such as copper wire, move in magnetic fields. Electrical currents can then be driven by these fields or voltages. This is how we get the power we use on a daily basis. Large generators, or “dynamos,” turn mechanical energy into electromagnetic energy, which powers our homes and workplaces, via this induction process. One of the most distinguishing characteristics of dynamos is that they require magnetic fields to function.
But, because there are no visible cables or large steel buildings in the cosmos, how can the fields form? Scientists began working on this topic around a century ago when they were trying to figure out where the Earth’s magnetic field came from. Seismic wave propagation research at the time revealed that much of the Earth was liquid under the colder surface layers of the mantle and that there was a core made of molten nickel and iron. Researchers thought that the Earth’s field was generated in some way by the convective motion of this hot, electrically conductive liquid and the rotation of the Earth.
Models eventually appeared that demonstrated how convective motion may magnify a pre-existing field. This is an example of “self-organization,” which occurs when large-scale structures emerge spontaneously from small-scale dynamics in complex dynamical systems. However, much like in a power plant, a magnetic field was required to create a magnetic field.
Throughout the cosmos, a similar mechanism is at work. However, the electrically conducting fluid in stars, galaxies, and the space between them is plasma, which is a state of matter that exists at extremely high temperatures and in which electrons are torn away from their atoms. Plasmas can be seen on Earth in the form of lightning or neon lights. The dynamo effect can magnify an existing magnetic field in such material if it starts at a low level.
Making the first magnetic fields
According to physicists, magnetic fields may be generated spontaneously by general large-scale movements as basic as sheared flows. On powerful supercomputers, Zhou constructed the underlying theory and ran numerical simulations. The plasma that occurs between stars and galaxies is extremely diffuse, which is an essential feature. Mechanical energy was turned into magnetic energy in the same way as it was in terrestrial instances. Although the origin of magnetic fields in the cosmos is unclear, Chinese scientists claim they have developed a self-consistent model for their formation on a cosmic scale.
Their next study will focus on determining if the technique can function on a time scale that is consistent with astronomical findings. “This study represents the first step in the construction of a new paradigm for understanding magnetogenesis,” the researchers write.
