Solar spots and storms are the most evident and spectacular indication of the frenetic activity that takes place every day on the incandescent surface of our star. They influence our planet, causing geomagnetic storms and auroras, as well as space weather throughout the solar system. And they are still a relatively mysterious phenomenon. We know, for example, that solar activity is linked to the Sun's magnetic field, but it is not certain where and how this is generated. A new study published in Nature helps to clarify this, revealing that the solar magnetic field probably arises in the outermost layers of the star, much further on the surface than previously thought.
The study was carried out by a group of physicists from various international universities. And it is based on the development of a model that simulates the appearance of the Sun's magnetic field based on the perturbations of the plasma that makes up the outermost layer of the star, the so-called convective zone. It is almost certain, in fact, that the magnetic field is generated by the interactions between the enormous currents of ionized particles (plasma) that circulate in this turbulent layer of the Sun, which makes up about a third of its entire mass, in a similar way to what happens in a dynamo.
“The basis for triggering a dynamo is that you need a region where there is a large amount of plasma moving in contact with other plasma, so that this shearing movement converts the kinetic energy into magnetic energy,” explains Keaton Burns, mathematician from MIT who participated in the research. “And until now it was thought that the solar magnetic field was created by movements that occur in the lower part of the convection zone.”
To study this kind of phenomena the only possibility is to simulate them with a supercomputer (since we cannot get closer to the actual Sun). And since it is an immense and extremely complex system, even using millions of hours of calculation on the most cutting-edge supercomputers, until now no one had managed to even come close to a credible simulation. For this reason, the authors of the new study decided to try to simulate a much smaller portion of the Sun: the outermost one.
The simulation was carried out using a program developed by Burns himself, called the Dedalus project, dedicated precisely to fluid dynamics simulations. And the algorithms used by Burns and colleagues identified patterns of plasma activity in the outer layers of the convective zone that can actually give rise to realistic solar activity. That's not all: the results obtained also made it possible to simulate the appearance of sunspots and solar minimums and maximums (that cycle of activity that increases and decreases every 11 years).
According to Burns, the discovery suggests that the Sun's magnetic field is not generated by a dynamo, but rather by a mechanism known as “magnetorotational instability”, present in the accretion disks surrounding black holes. It's too early to say whether the mathematician is right. But if the new simulation passes the scrutiny of the scientific community, the discovery could make it possible in the future to predict the arrival of solar storms with much more precision, and therefore to more effectively protect satellites and other technological infrastructures that are affected by their effects.