The sun’s magnetic field, dubbed its “heartbeat,” evolves in a cycle, the most prominent of which is the reversal of its polarities which takes places every eleven years, which coincides with the sunspot appearances.
Recent simulations have ushered in a major understanding of aspects of the sun’s magnetic field.
The interesting part of the findings of the simulation are these: at the base of the sun’s turbulent convection layer, torus-like bands of magnetic field of opposite polarities forms at mid-latitude on either hemisphere. Intriguingly, the torus-like bands of magnetic field undergo polarity reversals once every forty years.
How do we understand this? Basically, the sun functions as a dynamo. In an ordinary dynamo, a conducting coil rotates in a magnetic field which causes an electromotive force (emf) to be set up in the coil.
In a star, like the sun, there are no coils. The blobs of plasma, which are good conductors of electricity and rotating within the sun, take the role of coils. There is a magnetic field present there which induces an emf in these blobs, and this in turn stabilizes the magnetic field.
Now, it requires very advanced computer systems and very sophisticated numerical algorithms to actually model the sun. Such a simulation of the solar dynamo has been made by Paul Charbonneau of University of Montreal, Canada, and Piotr Smolarkiewicz of European Centre for Medium Range Weather Forecasts, UK.
In an article published in the journal Science (April 5, 2013), they describe a simulation which produces “zonal bands of a strong magnetic field [the torus-like bands] parallel to and antisymmetric about the equatorial plane” in the inner layers of the sun. This is the type of internal magnetic field believed to be conducive to the formation of sunspots.
The reversal of polarity of the torus-like bands every forty years is nearly four times as long as the observed solar cycle. The highlight of the study is that the researchers were able to show that a “regular cycle is produced.”
Despite the mismatch in timescales, “the very fact that a regular cycle is produced is already remarkable,” write the authors.
They further point out that other numerical simulations produce instead “large-scale zonal structures within the turbulent convection layer and peaking at low latitudes.”
These other simulations differ only in the details of simulation, “but evidently something essential is hiding in these details.”
The simulations by the two researchers also reveal factors that could be of significance in affecting the observed cycles.
They suggest that the accumulation of torus-like magnetic fields present beneath the convection layer “may be prone to magneto hydrodynamic instabilities.” This in turn could destabilize the cycles or affect the timing of polarity reversal.