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December 12, 2016
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Astronomers from the University of Hawaii Institute for Astronomy (IfA), Brazil, and Stanford University may have solved a long-standing solar mystery.
Two decades ago, scientists discovered that the outer five percent of the Sun spins more slowly than the rest of its interior. Now, in a new study, to be published in the journal Physical Review Letters, IfA Maui scientists Ian Cunnyngham, Jeff Kuhn, and Isabelle Scholl, together with Marcelo Emilio (Brazil) and Rock Bush (Stanford), describe the physical mechanism responsible for slowing the Sun's outer layers.
Team leader Jeff Kuhn said "The Sun won't stop spinning anytime soon, but we've discovered that the same solar radiation that heats the Earth is 'braking' the Sun because of Einstein's Special Relativity, causing it to gradually slow down, starting from its surface."
An image of the Sun taken with The Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory spacecraft. HMI is an instrument designed to study oscillations and the magnetic field at the solar surface, or photosphere. HMI observes the full solar disk with a resolution of 1 arcsecond.
The Sun rotates on its axis at an average rate of about once per month but that rotation isn't like, for example, the solid Earth or a spinning disk because the rate varies with solar latitude and distance from the center of the Sun.
The team used several years of data from the Helioseismic and Magnetic Imager on NASA's Solar Dynamics Observatory satellite to measure a sharp down-turn in the Sun's rotation rate in its very outer 150km. Kuhn said, "This is a gentle torque that is slowing it down, but over the Sun's 5 billion year lifetime it has had a very noticeable influence on its outer 35,000km." Their paper describes how this photon-braking effect should be at work in most stars.
The slowdown in the outer 100 kilometers is at least 2%, with the outer 30,000 km spinning about 5% slower overall. This can be compared to the trend in latitude, called solar differential rotation, where the poles rotate 30% slower than equator.
This change in rotation at the Sun's surface affects the large-scale solar magnetic field and researchers are now trying to understand how the solar magnetism that extends out into the corona and finally into the Earth's environment will be affected by this braking.
The research will appear in the January issue of Physical Review Letters, and is available online here.
Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Maunakea. The Institute operates facilities on the islands of Oahu, Maui, and Hawaii.