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 per cent of the Sun spins more slowly than the rest of its interior. Now, in a new study, 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 says, “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.”
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 disc because the rate varies with solar latitude and distance from the centre of the Sun.
The team used several years of data from NASA’s Solar Dynamics Observatory and the Helioseismic and Magnetic Imager instrument to measure a sharp down-turn in the Sun’s rotation rate in its very outer 150 kilometres (92 miles). Kuhn says, “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 35,000 kilometres (21747 miles).” Their research describes how this photon-braking effect should be at work in most stars.
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.