A new understanding of the solar dynamo published

Our resident solar expert, Dr. Leif Svalgaard, sends word of this new discovery.

Stanford solar scientists solve one of the sun’s mysteries

The sun’s magnetic field can play havoc with communications technology. Stanford scientists have now described one of the underlying processes that help form the magnetic field, which could help scientists predict its behavior.

By Bjorn Carey

Stanford solar scientists have solved one of the few remaining fundamental mysteries of how the sun works.  

The mechanism, known as meridional flow, works something like a conveyor belt. Magnetic plasma migrates north to south on the sun’s surface, from the equator to the poles, and then cycles into the sun’s interior on its way back to the equator.

The rate and depth beneath the surface of the sun at which this process occurs is critical for predicting the sun’s magnetic and flare activity, but has remained largely unknown until now.

The solar scientists used the Stanford-operated Helioseismic and Magnetic Imager (HMI) – an instrument onboard NASA’s Solar Dynamic Observatory satellite – to track solar waves in much the way seismologists would study seismic movements beneath the surface of the Earth. Every 45 seconds for the past two years, the HMI’s Doppler radar snapped images of plasma waves moving across the sun’s surface.

By identifying patterns of sets of waves, the scientists could recognize how the solar materials move from the sun’s equator toward the poles, and how they return to the equator through the sun’s interior.

“Once we understood how long it takes the wave to pass across the exterior, we determined how fast it moves inside, and thus how deep it goes,” said Junwei Zhao, a senior research scientist at the Hansen Experimental Physics Laboratory at Stanford, and lead author on the paper.

Although solar physicists have long hypothesized such a mechanism, at least in general terms, the new observations redefine solar currents in a few ways. First, the returning currents occur 100,000 kilometers below the surface of the sun, roughly half as deep as suspected. As such, solar materials pass through the interior and return to the equator more quickly than hypothesized.

More startling, Zhao said, is that the equator-ward flow is actually sandwiched between two “layers” of pole-ward currents, a more complicated mechanism than previously thought, and one that could help refine predictions of the sun’s activity.

“Considered together, this means that our previously held beliefs about the solar cycle are not totally accurate, and that we may need to make accommodations,” Zhao said.

For example, some computer models projected that the current solar cycle would be strong, but observations have since showed it is actually much weaker than the previous cycle. This inconsistency could be due to the previously unknown inaccuracies of the meridional circulation mechanism used in the simulations.

Improving the accuracy of simulations, Zhao said, will produce a better picture of fluctuations of the sun’s magnetic field, which can interfere with satellites and communications technology on Earth. The sun’s magnetic field resets every 11 years – the next reset will occur sometime in the next few months – and there is evidence that changes in the meridional flow can influence how the magnetic field evolves during a particular cycle.

“We want to continue monitoring variations of the meridional flow,” he said, “so that we can better predict the next solar cycle, when it will come and how active it will be.”

The report was published in the online edition of The Astrophysical Journal Letters. It was co-authored by three other researchers at the Hansen Experimental Physics Laboratory – senior scientists Rick Bogart and Alexander Kosovichev and research associate Thomas Hartlep – as well as NASA senior scientist Tom Duvall. Phil Scherrer, a professor of physics at Stanford, is the principal investigator of the HMI project and supervised the study.

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Leif adds an excerpt from the paper in an email:

Meridional flow inside the Sun plays an important role in redistributing rotational angular momentum and transporting magnetic flux, and is crucial to our understanding of the strength and duration of sunspot cycles according to flux-transport dynamo theories. At the Sun’s surface and in its shallow interior to at least 30 Mm in depth, the meridional flow is predominantly poleward with a peak speed of approximately 20 m/s.

The poleward plasma flow transports the surface magnetic flux from low latitudes to the polar region, causing the periodic reversals of the global magnetic field, a process important to the prediction of the solar cycles. The speed and variability of the meridional flow also play an important role in determining the strength and duration of the solar cycles, and the unusually long activity minimum at the end of Solar Cycle 23 during 2007–2010 was thought to be associated with an increase of the meridional flow speed during the declining phase of the previous cycle. Therefore, an accurate determination of the meridional flow profile is crucial to our understanding and prediction of solar magnetic activities.

Although the poleward meridional flow at the solar surface and in shallow depths has been well studied, the depth and speed profile of the equatorward return flow, which is expected to exist inside the solar convection zone to meet the mass conservation, largely remains a puzzle. It is generally assumed that the return flow is located near the base of the convection zone, although no convincing evidence had been reported. 

The continuous Doppler observations by the Helioseismic and Magnetic Imager onboard the recently launched Solar Dynamics Observatory mission (SDO) allow us to measure and detect the long-sought equatorward flow. Our analysis, which takes into account the systematic center-to-limb effect that was recently found in the local helioseismology analysis techniques, gives a two-dimensional cross-section picture of the meridional flow inside the nearly entire solar convection zone, and reveals a double-cell circulation with the equatorward flow located near the middle of the convection zone.

Figure 1 shows the new picture suggested by the HMI data.

This new picture of the solar interior meridional circulation differs substantially from the previously widely believed picture of a single-cell circulation with the equatorward flow near the bottom of the convection zone [the Conveyor Belt Model]. Through removing a systematic center-to-limb effect that was only recently identified, our analysis corrects and improves the previous solar interior meridional flow profile given by Giles (1999) using a similar analysis procedure.

The new meridional circulation profile poses a challenge to the flux-transport dynamo models, but provides more physical constraints to these models creating a new opportunity to further understand how magnetic field is generated and how magnetic flux is transported inside the Sun. Past dynamo simulations have already demonstrated that a meridional circulation profile with multiple cells might not be able to reproduce the butterfly diagram and the phase relationship between the toroidal and poloidal fields as observed, unless the dynamo model was reconsidered. However, on the other hand, solar convection simulations have shown the possibility of multi-cell circulation with a shallow equatorward flow (e.g.,Miesch et al. 2006; Guerrero et al. 2013), demonstrating that our analysis results are reasonable.

Moreover, a recent dynamo simulation, with the double-cell meridional circulation profile incorporated, showed that the solar magnetic properties could be robustly reproduced after taking into consideration of turbulent pumping, turbulent diffusivity, and other factors (Pipin & Kosovichev 2013). All these studies, together with our observational results, suggest a rethinking of how the solar magnetic flux is generated and transported inside the Sun.

Abstract: http://iopscience.iop.org/2041-8205/774/2/L29

pdf here: http://www.leif.org/EOS/ApJL-2013-Meridional-Flow.pdf