Like the Earth, the Sun likely has swirling polar vortices, according to new research led by the U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR). But unlike on Earth, the formation and evolution of these vortices are driven by magnetic fields.
The findings, recently published in the Proceedings of the National Academy of Sciences (PNAS), have implications for our basic understanding of the Sun's magnetism and the solar cycle, which could in turn improve our ability to predict disruptive space weather. The new research also paints a picture of what we might expect to see at the solar poles during future missions to the Sun and provides information that could be useful in planning the timing of such missions.
"No one can say for certain what is happening at the solar poles," said NSF NCAR senior scientist Mausumi Dikpati, who led the new study. "But this new research gives us an intriguing look at what we might expect to find when we are able, for the first time, to observe the solar poles."
The research was funded by NSF and NASA with supercomputing resources made available on NSF NCAR's Cheyenne and Derecho systems.
A mystery at the Sun's poles
The likely presence of some kind of polar vortices on the Sun does not come as a surprise. These spinning formations develop in fluids that surround a rotating body due to the Coriolis force, and they have been observed on the majority of planets in our solar system. On Earth, a vortex spins high in the atmosphere around both the north and south poles. When those vortices are stable, they keep frigid air locked at the poles, but when they weaken and become unstable, they allow that cold air to seep toward the equator, causing cold air outbreaks in the midlatitudes.
NASA's Juno mission returned breathtaking images of polar vortices on Jupiter, showing eight tightly packed swirls around the gas giant's north pole and five around its south. The polar vortices on Saturn, seen by NASA's Cassini spacecraft, are hexagonally shaped in the north pole and more circular in the south. These differences offer scientists clues into the makeup and dynamics of each planet's atmosphere.
Polar vortices have also been observed in Mars, Venus, Uranus, Neptune, and Saturn's moon Titan, so in some ways, the fact that the Sun (also a rotating body surrounded by a fluid) would have such features may be obvious. But the Sun is also fundamentally different from the planets and moons that possess atmospheres: the plasma "fluid" that surrounds the Sun is magnetic.
How that magnetism might influence the formation and evolution of solar polar vortices -- or whether they form at all -- is a mystery because humanity has never sent a mission into space that can observe the Sun's poles. In fact our observations of the Sun are limited to views of the face of the Sun as it points toward Earth and only offers hints at what might be transpiring at the poles.
A ring of vortices tied to the solar cycle
Since we have never observed the Sun's poles, the science team relied on computer models to fill in the blanks about what solar polar vortices might look like. What they found is that the Sun is likely to indeed have a unique pattern of polar vortices that evolves as the solar cycle unfolds and depends on the strength of any particular cycle.
In the simulations, a tight ring of polar vortices forms at around 55 degrees latitude -- the equivalent of Earth's Arctic circle -- at the same time that a phenomenon called the "rush to the poles" begins. At the maximum of each solar cycle, the magnetic field at the Sun's poles disappears and is replaced with a magnetic field of opposite polarity. This flip-flop is preceded by a "rush to the poles" when the field of opposite polarity begins to travel from about 55 degrees in latitude poleward.
After forming, the vortices head toward the poles in a tightening ring, shedding vortices as the circle closes, eventually leaving only a pair of vortices directly abutting the poles before they disappear altogether at solar maximum. How many vortices form and their configuration as they move toward the poles changes with the strength of the solar cycle.
These simulations offer a missing piece to the puzzle of how the Sun's magnetic field behaves near the poles and may help answer some fundamental questions about the Sun's solar cycles. For example, in the past many scientists have used the strength of the magnetic field that "rushes to the poles" as a proxy for how strong the upcoming solar cycle is likely to be. But the mechanism for how those things might connect, if at all, is not clear.
The simulations also offer information that may be used for planning future missions to observe the Sun. Namely, the results indicate that some form of polar vortices should be observable during all parts of the solar cycle except during the solar maximum.
"You could launch a solar mission, and it could arrive to observe the poles at completely the wrong time," said Scott McIntosh, vice president of space operations for Lynker and a co-author of the paper.
The Solar Orbiter, a cooperative mission between NASA and the European Space Agency, could give researchers their first glimpse of the solar poles, but the first look will be close to solar maximum. The authors note that a mission designed to observe the poles and to give researchers multiple, simultaneous viewpoints of the Sun could help them answer many long-held questions about the Sun's magnetic fields.
"Our conceptual boundary now is that we are operating with only one viewpoint," McIntosh said. "To make significant progress, we must have the observations we need to test our hypotheses and confirm whether simulations like these are correct."