By studying the dynamics of plasma turbulence, MIT researchers are helping to solve one of the mysteries of the origin of the cosmological magnetic field. One of the most profound mysteries in cosmology is the origin of the huge cosmic magnetic field. Although it is relatively weak, it has a great impact on the dynamics of the universe. Now, new research may finally discover the basic process of the origin of these mysterious cosmic magnetic fields.
When we gaze into space, all the astrophysical objects we see are surrounded by magnetic fields. This is true not only in the vicinity of stars and planets, but also in the deep space between galaxies and galaxy clusters. These magnetic fields are weak -- usually much weaker than those of refrigerator magnets -- but they are dynamic because they have a profound impact on the dynamics of the universe. Despite decades of intense interest and research, the origin of these cosmic magnetic fields remains one of the most fundamental mysteries in cosmology.
In previous studies, scientists have come to understand how turbulence, the agitation common to all types of fluids, amplifies pre-existing magnetic fields through so-called dynamo processes. But this extraordinary discovery only takes the mystery one step further. If the turbulence generator can only amplify the existing magnetic field, where did the first "seed" magnetic field come from?
Unless we understand how the seed magnetic field is generated, we will not have a complete and self consistent answer to the origin of astrophysical magnetic field. The new work carried out by MIT graduate student Muni Zhou, her tutor Nuno Loureiro (Professor of nuclear science and engineering at MIT) and colleagues at Princeton University and the University of Colorado at Boulder provided an answer. They explained the basic process of generating a magnetic field, that is, from a completely unmagnetized state to a state strong enough for the dynamic mechanism to take over and amplify the magnetic field to the extent we observed.
Magnetic fields are everywhere
Naturally occurring magnetic fields are everywhere in the universe. Thousands of years ago, they were first observed on the earth. Through their interaction with magnetized minerals (such as diamond), they were used for navigation before people had any understanding of their nature or origin. The magnetic force on the sun was discovered at the beginning of the 20th century because it affected the spectrum of light emitted by the sun. Since then, more powerful telescopes have discovered in the depths of space that these magnetic fields are everywhere.
Although scientists have long learned how to make and use permanent magnets and electromagnets, which have a variety of practical applications, the natural origin of the magnetic field in the universe is still a mystery. Recent work has provided some answers, but many aspects of this problem are still under debate.
Amplified magnetic field -- generator effect
Scientists began to think about this problem by considering how to generate electric and magnetic fields in the laboratory. An electric field is generated when a conductor, such as copper wire, moves in a magnetic field. These fields, or voltages, can then drive current. This is how the electricity we use every day is produced. Through this induction process, large generators or "generators" convert mechanical energy into electromagnetic energy to provide power for our homes and offices. A key feature of generators is that they require a magnetic field to operate.
But in the universe, there are no obvious wires or large steel structures, so how does the magnetic field come into being? Progress on this issue began about a century ago, when scientists were thinking about the source of the earth's magnetic field. At that time, the study of seismic wave propagation showed that below the colder surface of the mantle, most of the earth was liquid, and there was a core composed of molten nickel and iron. The researchers speculate that the convection of this hot, conductive liquid and the rotation of the earth combine in some way to produce the earth's magnetic field.
Finally, models emerged that showed how convective motion could amplify existing magnetic fields. This is an example of "self-organization" -- a feature often seen in complex dynamic systems -- large-scale structures grow spontaneously from small-scale dynamics. But just like in a power station, a magnetic field is always needed to create a magnetic field.
A similar process works throughout the universe. However, in stars and galaxies and the space between them, the conductive liquid is not molten metal, but plasma - a state of matter that exists at extremely high temperatures, in which electrons are pulled from their atoms. On earth, plasma can be seen in lightning or neon lights. In such a medium, the dynamo effect can amplify the existing magnetic field as long as it starts from a certain minimum level.
Make the first magnetic field
Where does this seed farm come from? This is the latest achievement of Zhou and her colleagues published on PNAs on May 5. Zhou developed the basic theory and carried out numerical simulation on a powerful supercomputer to show how the seed field is generated and what basic processes are at work. An important aspect of the plasma that exists between stars and galaxies is that it is very dispersed -- usually about one particle per cubic meter. This is very different from the situation inside stars, where the particle density is 30 orders of magnitude higher. Low density means that particles in cosmological plasma never collide, which has important effects on their behavior. These effects must be included in the models being developed by these researchers.
The calculations carried out by MIT researchers tracked the dynamics in these plasmas. They developed from orderly waves, but became turbulent as the amplitude increased and the interaction became strongly nonlinear. By including the detailed effects of small-scale plasma dynamics on macro Astrophysical Processes, they proved that the first magnetic field can be spontaneously generated by general large-scale motion as simple as shear flow. As in the case of the earth, mechanical energy is converted into magnetic energy.
An important output of their calculations is the amplitude of the expected spontaneous magnetic field. This shows that the magnetic field amplitude can rise from zero to the level at which the plasma is "magnetized" -- that is, the dynamics of the plasma is strongly affected by the presence of the magnetic field. At this point, the traditional generator mechanism can take over and raise the field to the observed level. Therefore, their work represents a self consistent model for generating magnetic fields on a cosmological scale.
Professor Ellen zweibel of the University of Wisconsin Madison pointed out that "although remarkable progress has been made in cosmology over the past few decades, the origin of the magnetic field in the universe is still unknown. It is wonderful to see the most advanced plasma physical theory and numerical simulation used to solve this basic problem".
Zhou and his colleagues will continue to improve their models and study the transition from the generation of seed fields to the amplification of generators. An important part of their future research will be to determine whether the process can work on a time scale consistent with astronomical observations. "This work provides the first step to establish a new paradigm for understanding magnetic generation in the universe," the researcher was quoted as saying.