Ultrafast waves discovered on the surface of the Sun defy explanation by current theories

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The interior of the Sun and stars is not accessible by conventional astronomy, especially with electromagnetic radiation. Nevertheless, the internal processes of stars can often be deduced on the basis of the activity of the surface of the star, through the study of the propagation of acoustic waves, like terrestrial seismology. We are talking here about helioseismology. Recently, scientists from New York University in Abu Dhabi analyzed 25 years of space and terrestrial data and detected mysterious waves. They move in the opposite direction to the rotation of the Sun and three times faster than expected by current solar physics. They put scientists in front of a new enigma! Explanations.

Just as seismology studies earthquakes, helioseismology studies sun tremors. Like almost all stars, the surface of the Sun is animated by oscillatory movements. These natural and permanent pulsations generate sound or gravity waves which propagate in the star and make it possible to study it from the surface towards the center. The source of the waves, in the case of the Sun, comes from surface turbulence. Millions of gas particles brought to a temperature of several thousand degrees stir and induce waves which propagate inside the Sun.

Some waves, rather tangential, stay close to the surface and are reflected at many points on the sphere. Others, more radial, reach the center of the Sun. Millions of different “modes” of vibration are generated every moment. The study of these modes informs us about the properties of stellar matter. With a diameter of 1 25 000 km, the Sun “rings” at a very low frequency, around 3 mHz, several tens of octaves below the accessible threshold to the human ear (ranging from 000 to 24 Hz).

However, acoustic waves are insensitive to parameters such as magnetic fields, turbulent viscosity and entropy gradients in the deep convection zone, which are critical contributions to theories of solar dynamics. Inertial waves can bridge this gap with their complementary sensitivities to these parameters. It is a type of hydrodynamic wave found in rotating fluid bodies. On Earth, inertial waves play an important role in weather systems, such as transporting cold polar air from the Arctic to North America. In the Sun, they probably play a role in transporting angular momentum to the equator, allowing the Sun’s equator to continue rotating faster than its poles.

This is how, by coupling traditional data and this new method of inertial waves, scientists from the space research center of New York University in Abu Dhabi were able to highlight a new type of wave. Their result is published in the journal Nature Astronomy.

The advent of helioseismology

This discovery follows a study published in 2021 by the same study center space, directed by Laurent Gizon and using helioseismology. The study led to the discovery of global oscillations of the Sun, comparable to the solar rotation period of 16 days. The oscillations manifest themselves on the solar surface as swirling motions with speeds of about five kilometers per hour. Similar to the study by Hanson et al. (2022), L. Grizon based himself on helioseismology, a young discipline in astrophysics. Indeed, it is only in 1960 that extremely weak pulsations of the Sun with a period of 5 minutes have were highlighted for the first time. By Doppler velocity measurement, they caused displacements of the photosphere at speeds of several kilometers per second.

But it was not until the following decade that understand, theoretically, this phenomenon as being the manifestation of the propagation of sound waves in the sun. Other observations, first by ground-based telescopes, before the advent of specially dedicated space missions, later confirmed both this discovery and the theory.

The properties of these oscillations, depending on the variations of the physical conditions and the movements of the interior of stars like the Sun, offer the possibility of probing a star in the same way as earthquakes inform geophysicists about the structure of the terrestrial globe. Laurent Grizon declared in a press release: “ The discovery of a new type of solar oscillation is exciting because it allows us to deduce properties, such as the force of convective driving, which ultimately control the solar dynamics”.

Unprecedented waves, of unexplained origin

From Then, Chris Hanson and his colleagues analyzed the data from 25 years of observations terrestrial telescopes of the Global Oscillation Network Group, as well as 000 years of observations of the HMI space telescope — acronym, in English, of Helioseismic and Magnetic Imager.

In the data, the researchers found a very coherent signal, which their analysis revealed as the presence of new waves, dubbed “retrograde waves at high frequency (HFR). They move against the direction of the Sun’s rotation and appear as a pattern of swirls on the star’s surface (or vortex), with antisymmetry between the North and South poles. They move on the surface of the sun at a speed three times higher than that established by the current theory on the movements of the plasma in the star. Their origin poses a real enigma.

Nevertheless, they would be similar to the magnetic Rossby waves, discovered in 2017, although much faster. You should know that Rossby waves appear in fluid flows, in rotating frames of reference, a situation in which the Sun finds itself. The Coriolis force — the force that deviates the trajectory of a moving object on the surface of a rotating object — is responsible for the appearance of this type of wave. Particularly on Earth, Rossby waves manifest themselves on very large scales in the form of undulatory movements of atmospheric or oceanic circulation. They were discovered at the beginning of the 20th century by the Swedish physicist Carl-Gustaf Rossby (1898-1957) who gave them his name.

Thus, in order to solve the riddle of the origin of these retrograde waves at high frequency, the scientists of the study assume that they would initially be excited by the Coriolis force. Then, three mechanisms could affect and modify the waves: 1) the magnetic fields inside the Sun; 2) the interaction with other ripples in the Sun, called gravity waves; 3) the compression of the plasma, that is to say the convection movements.

C. Hanson explained in a statement: “ If HFR waves could be attributed to any of these three processes, the discovery would have answered some of the unresolved questions that we still have on our star. However, these new waves do not seem to be the result of these processes, and this is exciting because it leads to a whole new set of questions

.

Similarly, high frequency waves were discovered in 1996, in the oceans, spreading up to four times faster than theoretically predicted, and they have proven very difficult to explain. Studying the similarities of the two phenomena via more detailed modeling could help explain the mystery of these waves in the future.

C. Hanson concludes on the probability that a combination of magnetism, gravity and compression is at the origin of these waves. He insists: “ It is very difficult to imagine a scenario where one of them does not play a role in improving the speed mode”.

By studying the internal dynamics of the Sun — thanks the use of waves — scientists can better appreciate the Sun’s potential impact on Earth and other planets in our solar system. The still undetermined nature of these waves promises new physics and new insight into solar dynamics.

2017Source: Nature Astronomy