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Waving goodbye to the Sun’s hot corona mystery?
Posted: 16 June 2011

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Incredibly fast waves travelling up to 2,000 kilometres per second have been observed in the Sun’s outer atmosphere by NASA’s Solar Dynamics Observatory (SDO). These waves could partially explain one of the biggest mysteries on the Sun: why the Sun’s outer atmosphere – known as the corona – is heated to temperatures of millions of degrees Celsius.

A medium sized solar flare coupled with a giant coronal mass ejection caught in action by the SDO on 7 June 2011. Such flare events produce high speed magnetic-acoustic waves. Image: NASA/SDO.

The waves had been predicted to exist in computer simulations of the Sun but until now they had eluded detection because there had been no telescope in existence that could capture them, until the arrival of the SDO. Onboard is the powerful Atmospheric Imaging Assembly (AIA), an instrument capable of observing the full disc of the Sun across seven different wavelengths ranging from optical through to ultraviolet light, and taking images down to a scale of 1,100 kilometres every 12 seconds with exposures of 0.1 to two seconds.

“It is the high temporal and spatial resolution of the AIA that enables us to see these waves clearly for the first time,” says Dr Wei Liu, a Stanford University Research Associate at Lockheed Martin’s Solar and Astrophysics Laboratory who led the research which is to be published in Astrophysical Journal Letters. When a solar flare erupts on the Sun, its energy sweeps the surrounding plasma into ‘magnetosonic waves’, which are a mixture of sound waves and magnetic ‘Alfven waves’ that begin to propagate outwards from the flare, racing up magnetic loops at velocities of 1,000 to 2,000 kilometres per second. They have periods of between half a minute and over three minutes, and wavelengths that stretch between 100,000 and 200,000 kilometres – the size of up to 15 Earths in a line.

Click here for a video of the fast moving waves propagating away from a flare region on the Sun. Credit: NASA/SDO/AIA.

Their importance to the solar corona mystery may be critical. “These waves must eventually dissipate and dump their energy into the corona,” Liu tells Astronomy Now. “The waves have a very large energy flux, 10,000 watts per square metre, comparable to the flux needed to heat the corona.”

That, however, is not the end of the mystery. SDO has seen about a dozen of these high speed, high energy waves over the year the spacecraft, but they have only been seen to be associated with the eruption of flares. Although the waves do heat the corona, they do not do so constantly. “They only contribute a very small amount of energy to the corona averaged over long periods of time,” says Liu. The puzzle, it seems, persists.