Magnetic field uses sound waves to ignite sun's ring of fire

Sound waves escaping the Sun's interior create fountains of hot gas that shape and power the chromosphere, a thin region of the sun's atmosphere which appears as a ruby red 'ring of fire' around the moon during a total solar eclipse, according to research funded by NASA and the National Science Foundation (NSF). These results were presented at the American Astronomical Society Meeting in Honolulu, Hawaii.

The chromosphere is important because it is largely responsible for the deep ultraviolet radiation that bathes the Earth, producing our atmosphere's ozone layer, and it has the strongest solar connection to climate variability. The new result also helps explain a mystery that's existed since the middle of the last century - why the chromosphere (and the tenuous corona above) is much hotter than the visible surface of the star. 'It's like getting warmer as you move away from the fire instead of cooler, certainly not what you expect,' said Scott McIntosh, a researcher at Southwest Research Institute, Boulder, Colo.

'This work finds the missing piece of the puzzle that has fascinated many generations of solar astronomers. When you fit this piece in place, our vision of the chromosphere becomes clear,' said Alexei Pevtsov, Program Scientist NASA Headquarters, Washington.

Using spacecraft, ground-based telescopes, and computer simulations, these new results show that the Sun's magnetic field allows the release of wave energy from its interior, permitting the sound waves to travel through thin fountains upward into the solar chromosphere. These magnetic fountains form the mold for the chromosphere.

'Scientists have long realized that solar magnetic fields hold the key to tapping the vast energy reservoir locked in the Sun's interior,' said Paul Bellaire, program director in NSF's division of atmospheric sciences. 'These researchers have found the ingenious way that the Sun uses magnetic keys to pick those locks.'

Over the past twenty years, helioseismologists have studied energetic sound waves as probes of the Sun's interior structure because they are largely trapped by the Sun's visible surface - the photosphere. The new research found that some of these waves can escape the photosphere into the chromosphere and corona.

To make the new discovery, the team used observations from the SOHO and TRACE spacecrafts combined with those from the Magneto-Optical filters at Two Heights (MOTH) instrument stationed in Antarctica, and the Swedish 1 meter (3 foot) Solar Telescope on the Canary Islands. The observations gave detailed insight into how some of these trapped waves manage to leak out through magnetic 'cracks' in the photosphere, sending mass and energy shooting upwards into the atmosphere above. 'The Sun's interior vibrates with the peal of millions of bells, but the bells are all on the inside of the building. We have been able to show how the sound can escape the building and travel a long way using the magnetic field as a guide,' continued McIntosh.

By analysing motions of structures in the solar atmosphere in detail, the scientists observed that near strong knots of magnetic field, sound waves from the interior of the Sun can leak out and propagate upward into its atmosphere. 'The constantly evolving magnetic field above the solar surface acts like a doorman opening and closing the door for the waves that are constantly passing by,' said Bart De Pontieu, a researcher Lockheed Martin Solar and Astrophysics Lab, Palo Alto, Calif.

These results were confirmed by state-of-the-art computer simulations that show how the leaking waves continually propel fountains of hot gas upward into the Sun's atmosphere, which fall back to its surface a few minutes later.

The scientists were able to independently demonstrate that the magnetic field controls the release of mass and wave energy into the solar atmosphere. The combination of these results demonstrates that a lot more energy can be pumped into the chromosphere by wave motions than researchers had previously thought. This wouldn't be possible without the relentlessly changing magnetic field at the surface.

The research team includes Stuart Jefferies, University of Hawaii, Maui, Hawaii; Scott McIntosh, Southwest Research Institute, Boulder, Colo.; Bart De Pontieu, Lockheed Martin, Palo Alto, Calif.; and Viggo Hansteen, University of Oslo, Norway and Lockheed Martin.