Scientists have discovered a class of never-before-seen waves inside the Sun that move in the opposite direction of its rotation and travel so fast that they defy explanation. The “as-yet undetermined nature of these waves promises novel physics and fresh insight into solar dynamics,” reports a new study.
The waves were serendipitously spotted by Chris Hanson, a research associate at New York University Abu Dhabi, and his colleagues, as the researchers scoured through decades of solar observations. The team was trying to answer a completely different question about the Sun when they noticed swirling patterns on its surface caused by these “high-frequency retrograde (HFR) waves,” according to a study published on Thursday in Nature Astronomy.
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“We weren’t intentionally looking for these waves,” said Hanson, who led the study, in an email. “One of the biggest mysteries of the Sun is the ‘convective conundrum’; where theory suggests, but observations cannot find, the existence of large convective cells,” which are also known as “giant cells.”
“We were therefore looking in the data for signatures of these cells and that’s when we discovered the HFR waves,” he continued. “Initially we thought they actually were ‘giant cells,’ but ruled these out subsequently (as stated in the paper).”
Hanson and his colleagues detected the waves by hunting through 24 years of images from the ground-based Global Oscillation Network Group (GONG) and 10 years of observations captured by the space-based Helioseismic and Magnetic Imager (HMI). While the HFR waves appear similar to a known phenomenon called Rossby–Haurwitz waves, they travel three times faster, a speed that cannot be explained by current models of the Sun.
“The novelty of this finding is that these distinct waves are traveling much faster than hydrodynamics alone can account for, hinting that profound insights into solar dynamics may stand to be gained,” said Hanson and his colleagues in the study.
Indeed, the researchers tried to make sense of the HFR waves as juiced-up versions of Rossby–Haurwitz waves, given the close resemblance of these two phenomena. The team considered the possibility that the waves were supercharged by interactions with the Sun’s intense forces—such as its magnetic fields, gravity, and compressibility—but ultimately these explanations came up short.
“Any of these possibilities would bring about new insight into the physics of the Sun’s interior; however, we have argued that all of these scenarios are unlikely,” Hanson and his colleagues said in the study. “There are evidently missing, or poorly constrained, ingredients in the standard models of the Sun, and determining the mechanism responsible for HFR modes will deepen our understanding of the interiors of the Sun and stars.”
To that end, the researchers plan to continue probing the possible origins of the waves with complex models of the Sun’s enigmatic interior, which cannot be directly observed with conventional telescopes. Solving this solar puzzle could shed light on a host of open questions about the structure, rotation, and physics of the Sun—and by extension, other stars.
“If we understand why and how these waves are set up, we can attempt to detect them in other stars,” Hanson said. “This in turn will lead to new insight into stellar structures.”
“The next step we are taking is to develop a numerical model to capture the behavior of inertial waves in the Sun,” he concluded. “As we add more and more physics to the model, the HFR waves may arise, telling us something new about the internal dynamics of the Sun.”
