Sunlight for brakes NASA/SDO

The sun is hoist with its own petard. Its outermost layer frustrates photons of light trying to escape. Now it seems that when light does eventually stream away, it may in turn slow down the sun’s rotation.

While the interior of the sun rotates like a solid sphere, the outer layers do not: the polar regions turn more slowly than the equator, and the layers closer to the core rotate more quickly than the edges.

The outermost 5 per cent of the sun lags the interior, and it’s not clear why. Even within the photosphere – the thin layer we see as the surface of the sun – there was a hint of a difference between the top layers and the bottom.


“The interior of the sun has complicated rotation because of turbulence, but the outer layer of the sun isn’t turbulent,” says Jeff Kuhn at the University of Hawaii in Pukulani. “The photosphere is stable, so it’s surprising that there would be any gradient in its rotation.”

We know that dust in interplanetary space is slowed by collisions with solar photons, and the resulting loss of angular momentum slowly moves the dust particles towards the sun. Inspired by this idea, Kuhn and his colleagues wondered if the gas particles in the outer layers of the sun experienced a similar slowing.

Using data from NASA’s Solar Dynamics Observatory, which has been orbiting the sun since 2010, Kuhn and his colleagues carefully measured the edge of the photosphere and its rotation to see if photons really did slow the star on their way out.

Photon pinball

Photons created in the sun’s interior bounce around on their way out, gaining a small amount of momentum from each atom they ricochet off. When a photon finally leaves the photosphere thousands to millions of years later, it carries that momentum with it.

As enormous numbers of photons radiate from the sun at different angles, the gas of the photosphere experiences a backward push. The push associated with each departing photon is minuscule, but over the sun’s 4.5-billion-year history, the barrage isn’t negligible. Kuhn’s team calculates that over that time it would cause a slowdown of about 3 per cent in the rotation of the outermost 100 kilometres of the sun – about 0.01 per cent of its radius – and exert a drag on the outermost 5 per cent.

“It is a little surprising, but if you think about how much energy the sun is radiating then you can begin to convince yourself that it really is a meaningful amount,” Kuhn says.

“I like the idea that photon emission can impart a significant torque on the outermost layers of the sun,” says Hugh Hudson at the University of California, Berkeley. “This basic idea seems to have been missed before.”

But Joergen Christensen-Dalsgaard at Aarhus University in Denmark questions whether such a small drag could account for the observed slowing of the outer layers, especially given the complexity of other effects within the sun which, he says, could swamp this one.

“Nevertheless, it is certainly a very interesting paper,” he says. “This is clearly entirely new, and probably unexpected, information about the dynamics of the sun’s outermost layers.”

Reference: arxiv.org/abs/1612.00873