Steve Tobias: Rediscovering the solar dynamo

I sat down for an hour with Steve Tobias a couple of years ago and told him about some of the correlations we’ve been finding between planetary and solar inertial motion, and solar activity levels. He listened attentively, but I don’t think I made too much of an impression. This press release heralds a new paper published in Nature by Tobias and Fausto Cattaneo.

Researchers at the Universities of Leeds and Chicago have uncovered an important mechanism behind the generation of astrophysical magnetic fields such as that of the Sun.

Scientists have known since the 18th century that the Sun regularly oscillates between periods of high and low solar activity in an 11-year cycle, but have been unable to fully explain how this cycle is generated.

The research, published in the journal Nature, explains how the cyclical nature of these large-scale magnetic fields emerges, providing a solution to the mathematical equations governing fluids and electromagnetism for a large astrophysical body.

The mechanism, known as a dynamo, builds on a solution to a reduced set of equations first proposed in the 1950s which could explain the regular oscillation but which appeared to break down when applied to objects with high electrical conductivity. The mechanism takes into account the ‘shear’ effect of mass movement of the ionized gas, known as plasma, which makes up the Sun. More importantly it 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,” said Professor Steve Tobias, from the University of Leeds’ School of Mathematics, a co-author of the research.

What is more, this mechanism could be used to describe other large, spinning astronomical bodies with large-scale magnetic fields such as galaxies.

The dynamo was developed through simulations using the high-performance computing facilities located at the University of Leeds.

“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.

The presence of spots on the Sun has been known since antiquity, and further analyzed after the invention of the telescope by Galileo in the 16th century. However, their cyclic nature, with periods of high activity (lots of sunspots) and low activity (few sunspots) following each other, was not identified until the 18th century. At the start of the 20th century it was then recognized that these sunspots were the result of the Sun’s magnetic field. Since then much effort has been devoted to understanding what processes lead to the formation of sunspots and the origin of their cyclic behavior.

“Shear-Driven Dynamo Waves at High Magnetic Reynolds Number,” by S. M. Tobias and F. Cattaneo, is published in the journal Nature, 23 May 2013:
http://www.nature.com/nature/journal/v497/n7450/full/nature12177.html

Abstract
Astrophysical magnetic fields often display remarkable organization, despite being generated by dynamo action driven by turbulent flows at high conductivity1, 2. An example is the eleven-year solar cycle, which shows spatial coherence over the entire solar surface3, 4, 5. The difficulty in understanding the emergence of this large-scale organization is that whereas at low conductivity (measured by the magnetic Reynolds number, Rm) dynamo fields are well organized, at high Rm their structure is dominated by rapidly varying small-scale fluctuations. This arises because the smallest scales have the highest rate of strain, and can amplify magnetic field most efficiently. Therefore most of the effort to find flows whose large-scale dynamo properties persist at high Rm has been frustrated. Here we report high-resolution simulations of a dynamo that can generate organized fields at high Rm; indeed, the generation mechanism, which involves the interaction between helical flows and shear, only becomes effective at large Rm. The shear does not enhance generation at large scales, as is commonly thought; instead it reduces generation at small scales. The solution consists of propagating dynamo waves, whose existence was postulated more than 60 years ago6 and which have since been used to model the solar cycle7.