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A new type of high-frequency acoustic wave discovered propagating on the Sun seems to be defying expectations. The waves appear on the surface of the Sun as a pattern of swirling vortices, moving against the Sun's rotation. The problem is that these high-frequency retrograde vorticity waves seem to be moving three times faster than predicted by theory – and solar physicists have been unable to determine why.
The discovery, they say, suggests that there is
new solar physics to be uncovered, as well as giving fresh insight into the
Sun's internal properties and activity. Although we can't actually see inside
the Sun, stars are remarkable in that their internal processes can often be
inferred based on surface activity.
In particular, acoustic waves can tell us a
lot. They are generated close to the surface and are then reflected, either
partially or entirely, towards the interior, where they resonate, creating
acoustic oscillations. Solar scientists study these oscillations to learn about
the interior of the Sun.
A team of scientists led by solar physicist
Chris Hanson of New York University, Abu Dhabi, studied and analyzed such data,
using 24 years of observations from the ground-based Global Oscillation Network
Group, and 10 years of observations from the space-based Helioseismic and
Magnetic Imager.
In the data, the researchers found a very
consistent signal, which their analysis reveals as the presence of previously
unseen waves. These formed a pattern of vortices on the surface of the Sun,
with antisymmetry between north and south poles, moving against the solar
rotation.
The fact that these waves are moving three times faster than expected, however, poses a conundrum. The team explored a number of possibilities for what was happening. First, the Coriolis force – the way a rotating spherical object's equator moves faster than its poles – excites vorticity waves, as we know can happen here on Earth. Then, there are three mechanisms that could affect and modify the waves: magnetism, gravity, or convection. None of them could account for the observation data, however.
"If the high-frequency retrograde waves could be attributed to any of these three processes, then the finding would have answered some open questions we still have about the Sun," Hanson says. "However, these new waves don't appear to be a result of these processes, and that's exciting because it leads to a whole new set of questions."
This suggests, the researchers say, that there
is missing or poorly constrained information in our models of the Sun that
resolving the mystery could fill in.
It also has relevance a bit closer to home. Scientists have found high-frequency waves in the ocean, propagating up to four times higher than predicted by theory, and which have proven very difficult to explain. Studying both phenomena together could help unravel the mystery behind them.
"The very existence of high-frequency retrograde modes and their origin is a true mystery and may allude to exciting physics at play," says physicist Shravan Hanasoge of New York University, Abu Dhabi. "It has the potential to shed insight on the otherwise unobservable interior of the Sun."
The research has been published in Nature Astronomy.