A team of researchers from the Department of Aeronautics and Astronautics at the University of Washington proposes a new model of plasma motion that would explain the 11-year sunspot cycle, magnetic reversals, and other previously unexplained solar phenomena.
NASA’s Solar Dynamics Observatory (SDO) captured this image of the Sun with sunspots in February 2013. The picture combines images from two SDO instruments: the Helioseismic and Magnetic Imager (HMI), which takes pictures in visible light that show sunspots, and the Advanced Imaging Assembly (AIA), which took an image in the 304 angstrom wavelength showing the lower atmosphere of the Sun, which is colorized in red. Image credit: NASA / SDO / AIA / HMI / Goddard Space Flight Center.
“Our model is completely different from a normal picture of the Sun. I really think we’re the first people that are telling you the nature and source of solar magnetic phenomena — how the Sun works,” said University of Washington’s Professor Thomas Jarboe.
The team’s model shows that a thin layer beneath the Sun’s surface is key to many of the features we see from Earth, like sunspots, magnetic reversals and solar flow, and is backed up by comparisons with observations of the Sun.
“The observational data are key to confirming our picture of how the Sun functions,” Professor Jarboe noted.
In the new model, a thin layer of magnetic flux and plasma, or free-floating electrons, moves at different speeds on different parts of the Sun.
The difference in speed between the flows creates twists of magnetism, known as magnetic helicity, that are similar to what happens in some fusion reactor concepts.
“Every 11 years, the Sun grows this layer until it’s too big to be stable, and then it sloughs off. Its departure exposes the lower layer of plasma moving in the opposite direction with a flipped magnetic field,” Professor Jarboe said.
When the circuits in both hemispheres are moving at the same speed, more sunspots appear.
When the circuits are different speeds, there is less sunspot activity. That mismatch may have happened during the decades of little sunspot activity known as the ‘Maunder Minimum.’
“If the two hemispheres rotate at different speeds, then the sunspots near the equator won’t match up, and the whole thing will die,” Professor Jarboe said.
“Scientists had thought that a sunspot was generated down at 30% of the depth of the Sun, and then came up in a twisted rope of plasma that pops out.”
Instead, the new model shows that the sunspots are in the ‘supergranules’ that form within the thin, subsurface layer of plasma that the study calculates to be roughly 100 to 300 miles (150-450 km) thick, or a fraction of the Sun’s 432,300-mile (695,700 km) radius.
“The sunspot is an amazing thing. There’s nothing there, and then all of a sudden, you see it in a flash,” Professor Jarboe said.
Other properties explained by the new model include flow inside the Sun, the twisting action that leads to sunspots and the total magnetic structure of the Sun.
“The paper is likely to provoke intense discussion. My hope is that scientists will look at their data in a new light and will have a new tool to understand what it all means,” Professor Jarboe said.
The team’s model is described in a paper in the journal Physics of Plasmas.
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T.R. Jarboe et al. 2019. The nature and source of solar magnetic phenomena. Physics of Plasmas 26 (9): 092902; doi: 10.1063/1.5087613
