New theory to explain why sun’s surface rotates slower than its core

Posted: February 6, 2017 by oldbrew in research, Solar physics

Credit: Imperial College London

Credit: Imperial College London

A small team of researchers with the University of Hawaii, Ponta Grossa State University in Brazil and Stanford University has found what they believe is the reason that the surface of the sun rotates more slowly than its core, reports

In their paper published in the journal Physical Review Letters, the team explains how they used a new technique to measure the speed of the sun’s rotation at different depths and what it revealed about the speed of the sun’s outer 70km deep skin.

Scientists have known for some time that the surface of the sun spins more slowly than its interior but have no good explanation for it. In this new effort, the researchers were able to take a better look at what was occurring and by doing so discovered what they believe is the source of the slowdown.

To gain a better understanding of what is happening with the sun, the researchers started with images collected by the Solar Dynamics Observatory—a probe that has been circling the sun since 2010. By processing three and a half years of images using filters, the researchers were able to get a detailed look at multiple layers of sun depth, which allowed them to calculate the circulation speed of each.

In looking at their overall results, they found that the outermost layer spun more slowly than all of those below it, which spun approximately 5 percent more than the rest of the photosphere.

Taking a cue from prior research that has shown that space dust is slowed as it collides with solar photons due to losses from angular momentum, the researchers created a model of the sun in which photons moving outward through interior layers of plasma eventually encounter plasma that is much less dense at its outermost layer.

As those photons collide with the plasma, which is moving, angular momentum is exchanged, which results in a net loss of plasma angular momentum. That net loss results in the plasma slowing as the photons that cause the slowdown escape into space.

The massive number of such collisions over the course of 4.5 billion years, the team theorizes, has resulted in the slower rate of spin of the outer layer that we observe today.

Source: New theory to explain why sun’s surface rotates slower than its core |

  1. p.g.sharrow says:

    well, they did pretty good until they got to “space dust” drag as the cause of the slowing. One wonders about the fudge factors required to make this compute? More likely, drag caused by the total solar system…pg

  2. pochas94 says:

    No mention of Coriolis forces? I’d think plasma rising toward the surface near the equator would cause the surface layer to slow.

  3. E.M.Smith says:

    Take a layer 100 km down moving at Velocity X,now have it rise to the surface, and it can still only do Velocity X unless something else accelerates it. That alone ought to make the surface move at fewer RPM than deeper down, since a R is longer but V constant.

    IMHO they ought to exhaust the available simple velocity and momentum physics before looking to pixie, er, space dust and friction in a vacuum…

  4. p.g.sharrow says:

    25 percent of the sun’s radius is gas. It’s real “surface” is at the 0.75 point of the solar radius. Lots of distance for slippage to manifest it’s self.
    Their lovely picture should be core–conductive zone–convective zone–radiative zone above the “Surface” in the plasma…pg

  5. oldbrew says:

    Note: the image in the post is not from the research, it’s from a London university.

  6. Tim Hammond says:

    Bit embarrassing for comments to talk about space dust – that’s not what they are saying. The loss of angular momentum is from photons colliding with plasma. The space dust was an analogy.

  7. oldbrew says:

    Tim H is correct. The wording in the report may have been misleading if read too quickly.

  8. Sparks says:

    We all know why the sun’s core moves faster than its surface, first of all the physics of matter having less distance to travel and second, the planets keep time with the core, the planets have more influence and interact with the part of the sun with the most mass.

    What this paper is trying to say is that the sun’s core slows down the surface as it releases relatively tiny amounts of mass over time to cause huge amounts of friction, this is more backassward science…

    “Scientists have known for some time that the surface of the sun spins more slowly than its interior but have no good explanation for it”

    Such a sweeping statement… Most of us have known for years, the physics and why the suns interior moves faster than the surface and still do, those who apparently don’t, have to come up with failed explanations over ‘n over again, just to toe the line of academic conformity…

    “Insanity: doing the same thing over and over again and expecting different results.” – Albert Einstein

  9. Brett Keane says:

    Is it really true that the core moves faster? Rather than just more revolutions? There are also the surface zonal differerces.
    It was understanding the sun’s gaseous composition that allowed for the possibility of noticeable planetary effects. That is, it is a fluid as far down as I can tell, though that leaves a lot unknown to me. Linnea, save us! LS too?

  10. oldbrew says:

    2007: Sun’s Ripples Reveal Clues to the Core

    The surface of the Sun rotates at different rates depending on location, with the equator spinning faster (about 25 days) than the poles (roughly 36 days).

    “The core of the Sun seems to rotate about three to five times faster on average,” Garcia told LiveScience.

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