A team of researchers with the University of Hawaii, Ponta Grossa State University in Brazil and Stanford University has found the reason behind the surface of the sun rotates more slowly than its core. 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.
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.