“Escaping Photons Slow Down the Surface Rotation of the Sun”
by Rebecca Boyle
Inside Science, February 7, 2017
I’m writing with a pitch for this week, based on a paper forthcoming in Physical Review Letters. It’s about one of my favorite topics, the sun, and one of its persistent mysteries: The fact that its surface rotates more slowly than its core. The answer may lie in its light, according to new research.
Particles of light escaping from the sun push back against the star’s outer layers, causing them to slow down. Sunlight itself is thereby causing the sun to spin more slowly.
The sun rotates on an axis like Earth does, but it does not do so gracefully. At its poles, a full rotation takes about 36 days, while at its equator, a full spin takes less than a month. And a similar process happens on its surface, where the top few layers rotate more slowly than the interior. These uneven rotations knot up the sun’s energy and magnetism, underlying the processes that form solar flares and superheat its atmosphere.
But the reasons for this slower spin have been unknown. Now, a new study suggests that at the surface, it’s just physics. As photons flee the surface of the sun, they carry away angular momentum, which pushes back the top 5 percent of the star and acts as a brake.
Angular momentum loss has been a solar suspect before. The sun constantly streams energetic particles into space in the form of the solar wind, for instance, and previous research suggested it could be to blame for the differential rotation at the equator and poles. But mathematical models show that the sun’s internal convection, and the fact that the sun is made of several layers, plays the key role in that strange scenario. Until now, no one had suggested the photosphere itself could contribute. To test whether this is possible, researchers from the US and Brazil used NASA spacecraft data to study the sun’s radial spin in the closest detail ever obtained. They say that the effect is small, but over 4.5 billion years, the angular momentum loss from light is just enough to account for the difference in rotation speed.
This isn’t embargoed, but is expected to be published Friday, Feb. 3. I got an email about it but I haven’t seen a press release anywhere (yet). I think this is really fascinating — the idea of light exerting a force is counterintuitive.
Please let me know what you think.