Topics: Astrophysics, Heliophysics, NASA, Solar System
|The Sun as seen by the Solar Dynamics Observatory (Courtesy: Solar Dynamics Observatory/NASA)
Sunlight is slowing the rotation of the Sun’s outermost layers by stealing its angular momentum. That is the claim of researchers in the US and Brazil who have studied acoustic waves oscillating through the Sun’s visible surface – the photosphere – to determine how fast the Sun spins at certain depths.
It has been known since the 1980s that the outer 5% of the photosphere rotates more slowly than deeper layers. However, solar physicists do not understand why this slowdown occurs, its total extent and its effect on the Sun’s magnetic dynamo and solar wind.
To solve this puzzle a team led by Ian Cunnyngham and Jeff Kuhn of the Institute for Astronomy at the University of Hawaii has observed acoustic waves at the limb (edge) of the Sun’s disc using the Helioseismic and Magnetic Imager on NASA’s Solar Dynamics Observatory (SDO), which orbits Earth.
The Sun is ringing like a bell as acoustic waves driven by turbulence crash through the plasma within its interior. The waves themselves, known as p-mode oscillations, have very low frequencies in the region of 3000 µHz and their harmonic patterns form the basis of helioseismology.
Cunnyngham and Kuhn’s team observed the oscillations at the solar limb, where the viewing angle makes it possible to determine how deep in the photosphere each oscillation is, allowing measurements of the rotation velocity at each depth. They found that the greatest amount of braking was occurring in the outer 70 km of the photosphere and that layers closest to the surface were rotating more slowly than deeper layers. This differential rotation could potentially twist localized magnetic field lines, affecting magnetic phenomena such as sunspots, active regions and even the formation of the solar wind.
Physics World: Photons are a drag on the Sun, Keith Cooper