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Published online 30 March 2022
A new kind of wave discovered in the Sun appears to travel much faster than theory predicts.
The best understanding of our Sun comes from studying the complex interaction of waves in its plasma, which are similar to those that drive circulation in the Earth’s atmosphere and oceans. These waves fall into three main types: acoustic soundwaves, gravity-induced waves, and inertial vorticity waves caused by the Sun’s spin.
Now, Chris Hanson and co-workers at New York University Abu Dhabi (NYUAD), UAE, have discovered a new kind of inertial wave, which travels at high speeds in the opposite direction from the Sun’s rotation. It is not yet clear what triggers these ‘high-frequency retrograde’ (HFR) waves, but the researchers are excited to learn more about them.
“One of the biggest mysteries of the Sun is the ‘convective conundrum’. Theory suggests there may exist large convective cells – outflows of hot material rather like the motions you see in water just before boiling point,” says Hanson. “We were looking for signatures of these cells, when we discovered the HFR waves.”
The team used more than 24 years of data from a global array of ground-based instruments called GONG, and the Helioseismic and Magnetic Imager onboard NASA’s Solar Dynamics Observatory spacecraft. Both missions provide Dopplergrams, which capture motions on the Sun’s surface, including oscillations caused by internal sound waves. Researchers use their knowledge of helioseismology to infer how these sound waves are altered by swirling vorticity flows; similar to the way geophysicists use waves to find oil in the Earth.
Hanson and colleagues used the high-performance computing resources at NYUAD to process the raw data and extract the sound wave information. A new technique called mode-coupling, developed at the team’s affiliated institutions in India and the US, enabled them to obtain a signal-to-noise ratio 10 times better than previous helioseismic methods.
“When we found the signal of the HFR waves, we saw that they moved three times faster than any other known inertial waves in the Sun,” says Hanson. The team also had to prove that the waves were traveling in the opposite direction to rotation, which is easier said than done.
“One of the difficulties with the Sun is that we only see half of it at any one time; the far side is hidden from our instruments.” This introduces artifacts in the data that make it difficult to distinguish between waves moving in opposite directions. Thanks to their high performance computers and mode-coupling, the team verified the waves’ direction and speed.
“Despite a rigorous literature search, we cannot explain the driving mechanism of these waves yet,” notes Hanson. So far, the team’s observations don’t fit with any known mechanisms, such as the coupling of rotational forces to gravity or magnetism.
“In the 1990s, oceanic scientists detected inertial waves named Rossby waves in the Earth’s oceans,” says Hanson. “These waves travel faster than theory would suggest, much like our HFR waves, but the reasons for this are still not conclusively known. It is worthwhile investigating whether the proposed mechanisms for ocean waves might apply to the Sun.”
Hanson, C.S., et al. Discovery of high-frequency retrograde vorticity waves in the Sun. Nat. Astron. http://dx.doi.org/10.1038/s41550-022-01632-z (2022).