Super-Resolution Making Order Out of Chaos

Friday, March 25, 2016

A paper was published today in Science by a team from the National Center for Atmospheric Research (NCAR), Chiba University in Japan, and the University of Tokyo. It is titled "Large-scale magnetic fields at high Reynolds numbers in magnetohydrodynamic simulations."

A high-resolution simulation of the movement of plasma in the sun (top) compared to a simulation of the star's magnetic field (bottom) reveals a close relationship. —Credit: Hideyuki Hotta, Chiba University.

New models of the Sun’s magnetic engine demonstrate that signature patterns in the magnetic field naturally dissolve and reappear as the resolution of the model is dramatically increased.

Over the past few decades models of the Sun’s interior have matured, showing that turbulent flows of charged gas, or plasma, that exist there naturally create a chaotic magnetic tangle. The issue is how the apparent order of solar magnetism, we have been recording observations of the Sun's face for hundreds of years, emerges from that mayhem to produce the apparently recurrent “solar cycle”.

Three-dimensional models of the Sun have been able to capture this large-scale order—which includes a predictable flip-flopping of the Sun's magnetic field about every 11 years—when run at a relatively low resolution. But something puzzling happens when researchers have tried to increase the model resolution wanting to explore the magnetic processes that unfold in the Sun at a much smaller scale: Those large-scale patterns, the yardstick of the solar cycle, can no longer be seen.

In a new study published in the journal Science, a team from the National Center for Atmospheric Research (NCAR), Chiba University in Japan, and the University of Tokyo, show that, for the first time, the Sun's large-scale variability can re-emerge when a model's resolution is pushed even further - to a scale finer than any ever attempted. To perform their bleeding-edge experiment, the scientists had to harness the power of two of the world’s largest supercomputers—NCAR's “Yellowstone” and the “K” computer at Japan’s Advanced Institute for Computational Science.

The experimental results give scientists important insight into how the Sun's magnetic fields, both tiny and massive in their scale, can co-exist and interact without destroying the solar cycle. Somehow, their experiment had to trick the physics of this complex system into behaving like it was the real Sun – they decided to miniaturize the spatial elements in the simulation to a level never attempted for a global simulation of the Sun. "It's like our model has to travel through this valley to get to the other side," said Matthias Rempel, a senior scientist at NCAR's High Altitude Observatory and a co-author of the paper. "Many other models of the same type are still on their way into the valley."

The existence of this conceptual valley is likely related to the fact that solar dynamos—the process by which the energy of turbulent flows of plasma is converted into magnetism—occur on both large and small scales inside the Sun. The large-scale solar dynamo is thought to be responsible for the solar cycle. But small-scale solar dynamos also exist, though their effects on the global scale are not understood well at all.

"There is a lot of small-scale turbulence on the Sun. The smallest eddies, or magnetic whirlpools, you find can be just meters, or even centimeters, in size," Rempel said. "The question is, when you have both large-scale and small-scale dynamos operating at the same time, how do they influence each other?"

To attack this problem Hideyuki Hotta, the lead investigator of Chiba University, pushed the simulation resolution across the valley to see if, somehow, communication between the magnetic scales could be re-established. Indeed, they did establish a connection between the global and local magnetic scales. While the models used in previous attempts could see the small-scale phenomena, it may be that they essentially couldn't see them well enough.

"In the past, the resolution was not high enough to really grow the small-scale component and see its full impact," Rempel said. "By pushing our model further than anyone else, we found that, in some way, the small-scale dynamo actually helps build the Sun's large-scale patterns."

**Link to article.