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The cost of shining so brightly

The emission of light particles extracts energy from the Sun’s outermost layer and causes it to spin more slowly

NASA/SDO, HMI Spots mark the surface of the Sun; the planet Venus is the dark circle in the upper leftNASA/SDO, HMI

The Sun appears to pay a price for providing nearby planets with light and heat. Its generosity costs it part of the energy that keeps it spinning around its axis, albeit very slowly. We still do not know what influence this could have on the life of the most important star in the neighborhood in the short- or very long term. Scientists suspect that the phenomenon could affect the magnetic fields generated inside the Sun, which are responsible for explosions that launch particles and energy into space and reach Earth’s surface, affecting satellite operation. However, further investigation is needed.

“The Sun will not stop spinning any time soon, but we discovered that the same solar radiation that warms the Earth is slowing down the Sun,” said astronomer Jeff Kuhn of the University of Hawaii, in a press release in December 2016.

Kuhn coordinated a study that precisely measured a subtle deceleration of the rotational speed of the Sun’s outermost layers. Published in February 2017 in the journal Physical Review Letters, the work was written together with Brazilian astronomer Marcelo Emilio, a professor at Ponta Grossa State University (UEPG) in the state of Paraná, and found that the layers furthest from the center of the star move more slowly than those closer to the center. Based on the measurements made to date, the researchers calculate that the photosphere, the atmospheric layer responsible for emitting the light particles that escape from the Sun’s surface, rotates at a speed about 2% lower than the star’s innermost layers.

This difference occurs because the Sun is not a solid sphere. It is made of a very hot gas (plasma) containing electrically charged ions of hydrogen and helium, the simplest and most abundant chemical elements in the Universe. We know that there is a difference in the density of this plasma in accordance with its depth. It is denser in the regions nearer the Sun’s core than those far away—in the star’s core, the plasma is 10 times denser than lead, while in the Sun’s atmosphere it is 10,000 times less dense than the air on the Earth’s surface.

Because of this density gradient, the innermost two-thirds of the Sun act like a solid sphere, in which all points rotate together in the same direction. “The core spins as a unit and it would be difficult for us to measure any deceleration,” says astrophysicist Nelson Leister, a professor at the University of São Paulo (USP) and a specialist in solar physics and astronomy.

The outermost third, in turn, acts like a fluid: it moves in the same direction as the innermost region, but at different speeds. Measurements made in recent decades had already indicated that the outermost third of the Sun moved more slowly than the innermost two thirds. Now we also know that, in the surface layer, speeds fall with distance from the Sun’s core.

For some time astronomers have suspected that, even in the Sun’s atmosphere, different strata move at different speeds. Data obtained in the 1980s on the displacement rate of Sun spots—darker, colder regions in the Sun’s atmosphere—showed small variations, interpreted by some researchers as the result of a possible difference in the speed at which they moved.

Using 27 million ultra-high resolution images of the Sun, obtained by the NASA Solar Dynamics Observatory (SDO) satellite, Kuhn, Emilio and their collaborators measured the subtle changes in the movement of the photosphere with a precision never before obtained. They confirmed that this difference in rotational speed exists and can be measured even between strata in a 150-kilometer band of this layer—these 150 kilometers correspond to 0.02% of the Sun’s radius.  A point at the top (outermost edge) of this layer takes 2.7% more time to complete a rotation around the Sun’s axis than a point at the same latitude, but located at the bottom (innermost edge). It is a subtle effect, but becomes important over time.

A longer trip
An example can help clarify what happens. Since the 17th century, we have known—through the study of sun spots—that rotation speed depends on latitude, with lower speeds at the poles. If a point in an internal layer of the star takes 30 days to rotate once around the Sun’s axis, a point at the bottom of the photosphere located at the same latitude takes 19 hours more to complete the rotation. A point at the top of the photosphere takes an additional two hours, for a total of 21 hours.

Astronomers estimate that, since the Sun emerged 5 billion years ago, it has already completed 60 billion rotations around its axis. In all this time, the drag of the atmosphere must have been sufficient to change the rotational speed of the outermost 5% of the Sun, equivalent to a depth of 35,000 kilometers. This data, presented in the article in Physical Review Letters, agrees with previous studies. In the late 1980s, measurements of the propagation speed of sound waves inside the Sun, carried out using a technique called helioseismology, indicated that the outermost 5% of the star rotated more slowly than the rest. But no one could explain why.

In their current paper, Emilio, Kuhn’s team, and Stanford astronomer Rock Bush, creator of one of the SDO instruments, propose that the deceleration of the Sun is a consequence of photon drag. It is as if each photon (particle of light) emitted by the outermost layers of the star carries with it a tiny amount of the energy that maintains the rotation of the solar atmosphere. According to Kuhn, this effect is slowing the star very slowly, from its surface inward.

This hypothesis can only be confirmed with the passage of time. “This loss is very small,” says Emilio, director of the UEPG Astronomical Observatory. According to the researcher, who carried out the most precise measurement ever of the Sun’s radius (679,000 km, 109 times larger than Earth’s) in 2012 together with colleagues, we would have to observe the Sun for millions of years to measure whether the loss of photons corresponds to the deceleration that is taking place.  Or, at least await the completion of the largest telescope designed for solar observation: the Daniel K. Inouye Solar Telescope (DKIST), under construction on a Hawaiian island. Scheduled to be ready in 2020, it will incorporate a 4-meter diameter mirror that will have to be cooled in order to withstand so much energy from the Sun. “When it is operating,” says Emilio, “this telescope will observe part of the Sun with much greater resolution and perhaps will be able to make this type of measurement with great precision.”

Scientific article
CUNNYNGHAM, I. et al. A Poynting-Robertson-like drag at the Sun’s surface. Physical Review Letters. February 3, 2017.