May 23, 2013
Magnetic Field Model Of Sun Explains Solar Activity Cycles
April Flowers for redOrbit.com - Your Universe Online
Since the 18th Century, scientists have been aware that the Sun oscillates between periods of high and low solar activity in an 11-year cycle. So far, though, they have been unable to fully explain how this cycle is generated.
An important mechanism behind the generation of astrophysical magnetic fields such as that of the Sun has been uncovered by researchers at the Universities of Leeds and Chicago. The findings of their study, published in the journal Nature, explain how the cyclical nature of these large-scale magnetic fields emerges. The research also provides a solution to the mathematical equations governing fluids and electromagnetism for a large astrophysical body.
The identified mechanism is known as a dynamo. It builds on a solution to a reduced set of equations first proposed in the 1950s that could explain the regular oscillation but which appeared to break down when applied to objects with high electrical conductivity. The dynamo takes into account the “shear” effect of mass movement of plasma, the ionized gas that makes up the Sun. More importantly, the dynamo mechanism does so in the extreme parameter regime that is relevant to astrophysical bodies.
"Previously, dynamos for large, highly conducting bodies such as the Sun would be overwhelmed by small-scale fluctuations in the magnetic field. Here, we have demonstrated a new mechanism involving a shear flow, which served to damp these small-scale variations, revealing the dominant large-scale pattern", Professor Steve Tobias, from the University of Leeds' School of Mathematics, said in a statement.
Other large, spinning astronomical bodies with large-scale magnetic fields such as galaxies can also be described with this mechanism.
Simulations using high-performance computing facilities at the University of Leeds were employed by the scientists to develop the dynamo.
"The fact that it took 50 years and huge supercomputers shows how complicated the dynamo process really is." said Prof Fausto Cattaneo, from the University of Chicago's Department of Astronomy and Astrophysics.
In an unconnected, yet related, study also published in Nature, an interdisciplinary research team led by a Johns Hopkins mathematical physicist says it has found a key to the mystery of why the magnetic fields of some solar flares do not behave the way they should, as dictated by the flux-freezing theorem.
When a solar flare erupts from the Sun, like we have experienced this week, the magnetic field of the flare sometimes breaks a widely accepted rule of physics: the flux-freezing theorem. The theorem dictates that the magnetic lines of force should flow away in lock-step with the particles, whole and unbroken. Instead of behaving this way in every instance, sometimes the lines break apart and quickly reconnect.
The research group suggests that the culprit is turbulence - the same sort of violent disorder that can jostle a passenger jet when it occurs in the atmosphere. The team used complex computer modeling to imitate what happens to magnetic fields when they encounter turbulence within a solar flare. These models allowed the researchers to build their case, explaining why the usual rule did not apply.
“The flux-freezing theorem often explains things beautifully,” said Gregory Eyink, a Department of Applied Mathematics and Statistics professor at Johns Hopkins University (JHU). “But in other instances, it fails miserably. We wanted to figure out why this failure occurs.”
Hannes AlfvÃ©n, who later won a Nobel Prize in physics, developed the flux-freezing theorem 70 years ago, which states that magnetic lines of force are carried along in a moving fluid like strands of thread cast into a river, and thus they can never “break” and reconnect. However, scientists have discovered that this principle does not always hold true during violent solar flares. Previous studies have shown that sometimes the magnetic lines break like stretched rubber bands and reconnect in as little as 15 minutes. Even a short duration like that releases vast amounts of energy that power the flare.
“But the flux-freezing principle of modern plasma physics implies that this process in the solar corona should take a million years!” Eyink said. “A big problem in astrophysics is that no one could explain why flux-freezing works in some cases but not others.”
Scientists suspected that turbulence was playing havoc with the behavior predicted by the flux-freezing theorem. Eyink teamed up with other experts in astrophysics, mechanical engineering, data management and computer science to find out. “By necessity, this was a highly collaborative effort,” Eyink said. “Everyone was contributing their expertise. No one person could have accomplished this.”
Eyink and his team developed a computer simulation to replicate what happens under various conditions to the charged particles that exist in a plasma state of matter within solar flares.
“Our answer was very surprising,” Eyink said. “Magnetic flux-freezing no longer holds true when the plasma becomes turbulent. Most physicists expected that flux-freezing would play an even larger role as the plasma became more highly conducting and more turbulent, but, as a matter of fact, it breaks down completely. In an even greater surprise, we found that the motion of the magnetic field lines becomes completely random. I do not mean ℠chaotic,´ but instead as unpredictable as quantum mechanics. Rather than flowing in an orderly, deterministic fashion, the magnetic field lines instead spread out like a roiling plume of smoke.”
Eyink said that although some scientists may still believe that there are other explanations for solar flares, he thinks that his team “made a pretty compelling case that turbulence alone can account for field-line breaking.”
“We used ground-breaking new database methods, like those employed in the Sloan Digital Sky Survey [SDSS], combined with high-performance computing techniques and original mathematical developments,” he said. “The work required a perfect marriage of physics, mathematics and computer science to develop a fundamentally new approach to performing research with very large datasets.”
The findings could lead to a better understanding of solar flares and mass ejections of material from the sun´s corona, said Eyink, because such powerful “space weather” or geomagnetic storms can endanger astronauts, knock out communications satellites and even lead to massive blackouts of electrical power grids on Earth.