Researchers employ Kraken to study solar magnetism, stellar evolution
by Gregory Scott Jones
To the naked eye, the Sun comes and goes each day exactly as it appeared the day before—round, yellow-orange, and bright, perhaps mankind’s most reassuring and longest-running companion.
Figure 1: Animation of a young, Sun-like star undergoing a cyclic reversal of its magnetic fields. This simulated star is rotating five times faster than our Sun. Visualization produced using VAPOR, developed by the National Center for Atmospheric Research.
Surprisingly, however, there is much we still don’t know about our closest star, and despite its uniformity from our window more than 90 million miles away, the Sun is anything but constant. Up close it is a tumultuous, violent, evolving creature wrapped in radiation and magnetism.
And despite the Sun’s reassuring appearance, its magnetic forces can wreak havoc on communications satellites, and solar flares might one day affect space weather, no doubt a critical factor in any future long-term space travel. Therefore understanding the Sun’s inherent magnetism and its evolution will not only help us to understand the heartbeat and evolution of our solar system, but could also prove useful in the very future of mankind.
To this end a team led by Juri Toomre from the University of Colorado is using Kraken, a Cray XT5 supercomputer, to simulate a Sun-like star. Located at the National Institute for Computational Sciences, a National Science Foundation-funded supercomputer managed by the University of Tennessee and located at Oak Ridge National Laboratory, Kraken is ranked eighth on the Top 500 list of the world's fastest supercomputers.
The team’s findings are revealing much about our most familiar stellar companion. Perhaps even more important, however, are the questions being raised that will require enhanced computing power to answer and thus fully understand the center of our solar system. According to team member Ben Brown (Dept. of Astronomy and Center for Magnetic Self-Organization, University of Wisconsin), researchers want to know in exactly what region of the Sun the magnetic fields originate and how the Sun’s layers of density, known as stratification, and its rotation speed create and influence those magnetic fields.
Solar Anatomy 101
The Sun’s magnetism is literally written on its face, in the form of sunspots. These visible black spots are throbbing with magnetic energy and are responsible for stopping convection and cooling the area they occupy. For researchers they are invaluable—the magnetic activity within sunspots can actually be measured, providing valuable information, such as the 11-year cycle of the Sun’s magnetic fields, as evidenced by the cyclical appearance of sunspots over 11-year periods.
And while these spots appear relatively close to the solar surface, researchers know that because of solar dynamo theory, which states that the Sun’s magnetic fields are generated by an electric current flowing deep inside the sun, the magnetic fields must likewise originate somewhere in the interior of the Sun. The location and mechanism of the dynamo, or the generator of the magnetic fields, remains a mystery however, said Brown.
For years it was assumed that the dynamo lie in the tachocline, or the region of the Sun between the radiative zone, near the core, and the convective zone, which occupies the outer 30 percent. Dynamo theory holds that the fields are generated out of fluid motion, and the inner part of the Sun, particularly the radiative zone, is stratified, meaning it lacks the fluid motion necessary to generate the fields. However, the convective zone was thought to be too turbulent to provide the stability necessary to maintain the fields.
Therefore theory held that the tachocline must be home to the Sun’s magnetic personality, even though it would be hard to get the fields to the surface via the chaotic convective zone. Brown’s numerous simulations surprisingly revealed the magnetic fields originating in the convective zone, a paradigm-shifting development in solar science. “Somewhere in or near the convective zone has to be the location of the solar dynamo, but we have no idea how it works,” said Brown.
A Magnetic Personality
Another phenomenon investigated by the team was the relationship between rotation, stratification (layers of density), and the magnetic fields. “Do we get any large scale structures [magnetic fields],” asked Brown hypothetically. The answer, he said, is yes, but with caveats, such as the properties of the simulated star.
For starters, the team simulated a star with much less turbulence than the Sun, due to the computational limits imposed by turbulence. To accurately simulate the Sun, the resolved turbulence of the simulations would need to be ramped up by six orders of magnitude, which, said Brown, would take 80-100 years of computational progress according to Moore’s Law, which states that computers roughly double in speed every two years.
Furthermore, the team used simulated stars that rotated 3 to 5 times faster than the Sun does now. Generally, the faster a star rotates, the more magnetic its personality. “When we observe different stars we notice they rotate at different rates, and the ones that rotate faster are more magnetic, but we don’t know why,” said Brown.
Despite the decreased turbulence and the enhanced rotation, the simulated stars were about as close to the Sun as is computationally possible. “We can’t simulate the Sun,” said Brown, “but we can make something similar to it in many respects and test whether or not our decisions are valid.” And when you do that, you can reproduce certain observed features of the Sun and match them at some level of detail in future simulations.
One particularly surprising finding regarding the relationship between rotation and magnetism stands out: the stars rotating more rapidly display stronger differential rotation, or different rates of rotation at the poles and equator, which produces global scale magnetic fields, as are observed in the Sun. For researchers this means that their model so far is in line with observation, and they are getting closer to unraveling the mysterious properties of our stellar next door neighbor. In other words, they can confidently proceed and continue to increase the resolution of their simulations, all the while obtaining more and more information as to the source of the Sun’s magnetic “dynamo” personality.
To Brown, “major salient feature's” of these models include that the team built magnetic fields in the convection zone which remained for an extended period of time, much like the Sun’s fields, and that these fields can undergo regular global scale reversals (demonstrated by the movie) that show similarities to the sun's own eleven year cycles of activity. However, he admits that the work on the mechanism of the dynamo was “a very crude first attempt.” In the end, the ultimate goal of the simulations, according to Brown, was to “test ideas and see what they tell us about the solar dynamo.”
Test ideas they did, and while perhaps more questions than answers were raised, at least the team knows it’s headed in the right direction. And as they refine their models and up the resolution of their simulations, they bring humankind one step closer to a fuller understanding of its most important star, and maybe, just maybe, help save a satellite or land a man on Mars.