By studying the dynamics of plasma turbulence, MIT researchers are helping to solve the mystery of the origins of magnetic fields
MIT Technology Review Italy
In the universe, all visible astrophysical objects are embedded in magnetic fields, typically much weaker than those of a refrigerator magnet, but dynamically significant in the sense that they have profound effects in the dynamic realm. In previous research, scientists have come to understand how turbulence, the seething motion common to fluids of all kinds, could amplify pre-existing magnetic fields through so-called dynamo theory. But this discovery only moved the question: where does the initial magnetic field come from in the first place?
The answer appears to come from the work of Muni Zoho and Nuno Loureiro of MIT’s Department of Nuclear Science and Engineering and colleagues at Princeton University and the University of Colorado at Boulder, who show how a field is generated from a completely non-magnetized state to the point where it is strong enough for the dynamo mechanism to take over and amplify the field to the observed magnitudes.
Scientists began to think about this problem by considering how electric and magnetic fields were produced in the laboratory. When conductors, such as copper wire, move in magnetic fields, electric fields are created, which are capable of driving electric currents. Through this induction process, at the base of electricity, large generators or “dynamos” convert mechanical energy into electromagnetic energy that powers our homes and offices. A key feature of dynamos is that they need magnetic fields to function.
But there are no obvious wires or large steel structures in the universe, so how do fields arise? Advances on this issue began about a century ago when scientists pondered the source of the Earth’s magnetic field. Studies on the propagation of seismic waves, dating back over a century, showed that much of the Earth, beneath the colder surface layers of the mantle, was liquid and that there was a core composed of molten nickel and iron. The researchers theorized that the convective motion of this hot, electrically conductive liquid and the rotation of the Earth somehow combined to generate the Earth’s field..
Eventually, models emerged that showed how convective motion could amplify an existing field. This is an example of “self-organization” – a feature often seen in complex dynamical systems – in which large-scale structures grow spontaneously from small-scale dynamics.. But just like in a power plant, a magnetic field was needed to create a magnetic field.
A similar mechanism is at work throughout the universe. However, in stars and galaxies and in the space between them, the electrically conducting fluid is not molten metal, but plasma, a state of matter that exists at extremely high temperatures, in which electrons are torn away from their atoms. On Earth, plasma can be seen in lightning or neon lights.
The recent work of Zhou and his colleagues, published in “PNAS”, ran numerical simulations on powerful supercomputers to test this theory. The plasma found between stars and galaxies is extraordinarily diffuse, typically about one particle per cubic meter. The situation is very different in the interior of the stars, where the particle density is about 30 orders of magnitude higher. Low densities mean that particles in cosmological plasmas never collide.
The MIT researchers’ calculations followed the dynamics in these plasmas, which developed from well-ordered waves, but became turbulent as amplitude increased and interactions became nonlinear. Extending the detailed effects of small-scale plasma dynamics to macroscopic astrophysical processes, Scientists have shown that the first magnetic fields can be produced spontaneously through generic large-scale movements as simple as translated flows. Just as in the case of the terrestrial examples, mechanical energy has been converted into magnetic energy.
An important result of their calculation was the magnitude of the spontaneously generated magnetic field, which could rise from zero to a level where the plasma is “magnetized”. At this point, the traditional dynamo mechanism can take over and bring the fields to observed levels. Their work will have to prove to be valid on a time scale consistent with astronomical observations, but it already provides, as the authors said, “the first step in building a new paradigm for understanding magnetogenesis in the universe”.
Image by Luminas Art from Pixabay
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