30 June 2020
Flow of Sun’s plasma controls sunspot cycle
Published online 26 June 2020
A helioseismology study maps the flow of plasma inside the Sun and supports a model that also explains the sunspot cycle.
The flow of plasma in the solar interior is critical for explaining the generation of the Sun’s magnetic field and the associated 11-year sunspot cycle. However, it has been difficult to determine the geometry of the ‘meridional flow’ of plasma from the poles to the equator below the Sun’s surface, and whether this flow is linked to the sunspot cycle.
Results from a ten-year helioseismology study conducted by Laurent Gizon and co-workers at the Max Planck Institute for Solar System Research in Göttingen, Germany and New York University Abu Dhabi, UAE show that the meridional flow forms a single closed loop, or cell, in each hemisphere. The plasma is carried towards the equator at the base of the convection layer (the outer 29% of the Sun), and back to the pole on the surface.
“Helioseismologists use sound waves to probe the Sun's interior, in the same way geophysicists use the seismic waves from earthquakes to probe the Earth,” says Gizon. “In this study, we observed sound waves propagating in the north-south direction through the solar interior. These waves sense the presence of the meridional flow: they travel faster along the flow than against it.”
The team measured tiny perturbations in the travel times of the sound waves to reconstruct the geometry of the flow. They verified their results using data from two independent, overlapping observational datasets: one from the SOHO spacecraft (ESA/NASA) and the other from the ground-based GONG project. The datasets cover two full sunspot cycles, from 1996 to 2019. Interestingly, the results support a popular theory that explains the equatorial drift of the locations at which sunspots emerge through the 11-year solar cycle.
“We found that plasma makes a full turn around the convection zone in approximately 22 years,” says Gizon. “Sunspots form when the magnetic field rises from the solar interior and pierces through the solar surface. Sunspots emerge at different latitudes at different times, and appear closer to the equator as the solar cycle progresses.”
This latitudinal drift of sunspots is explained by the meridional flow dragging the magnetic field toward the equator at the base of the convection zone.
“These results, if they hold up to further scrutiny, will resolve the biggest observational discrepancy in our most promising physical model for the solar cycle,” says physicist Paul Charbonneau of the Université de Montréal, Canada. “The determination of the latitudinal flow speed at the base of the convection zone is particularly interesting for future modeling.”
Gizon, L. et al. Meridional flow in the Sun’s convection zone is a single cell in each hemisphere. Science 368, 1469–1472 (2020).