News in Science

Solar tornadoes help solve coronal mystery

Hot twister Scientists say magnetic super tornadoes, thousands of times bigger than anything on Earth, may explain why the Sun's outer atmosphere is millions of degrees hotter than its visible surface.

The Sun has a surface temperature of about 5500°C, however it's outer atmosphere or corona, reaches temperatures of two million degrees.

For almost a century astronomers have been trying to understand why temperatures in the Sun's atmosphere get hotter rather than colder, the further away you are from its surface.

Now scientists including Dr Sven Wedemeyer-Bohm from the University of Oslo have linked magnetic super tornadoes to this coronal heating.

Wedemeyer-Bohm and colleagues were studying magnetic ring like structures on the Sun known as chromospheric swirls, which resemble giant tornadoes.

These super tornadoes have foot prints about 100 kilometres wide on the Sun's surface or photosphere, which can funnel out to diameters of more than 5500 kilometres, sometimes with cyclone like spiral arms.

Hot clue

Using NASA's Solar Dynamics Observatory spacecraft, they detected very hot plasma in the corona at the same location as the swirls.

Writing in the journal Nature, Wedemeyer-Bohm and colleagues report seeing these super tornadoes linking the solar surface or photosphere with the corona, thousands of kilometres higher up.

They then used computer models to determine that energy was flowing from the photosphere to the corona.

"We observed plasma (ionized gas) moving at speeds of over 10,000 kilometres per hour in these super tornadoes," says Wedemeyer-Bohm.

"Our computer models suggest that the speeds could be even higher than what we are able to observe so far."

Magnetic energy

The tornadoes which last an average of 12 minutes, are formed by the convective motion of super heated plasma near the photosphere.

"As cooler plasma sinks into the Sun's interior, it creates a swirling effect like water draining out of a bathtub," says Wedemeyer-Bohm.

"As it plunges down, it drags magnetic field lines with it, causing them to rotate and generate Alfven waves, which can be understood as a magnetic analogue of sound waves."

"These Alfven waves travel upwards along the magnetic field in the tornado and are transformed into heat in the corona," says Wedemeyer-Bohm.

There are an estimated eleven thousand of these super tornadoes on the Sun's surface at any one time.

Wedemeyer-Bohm and colleagues conclude, that's probably enough to transfer the energy needed to heat the corona to temperatures of several million degrees.

"We have many ideas what to do next," says Wedemeyer-Bohm.

"An obvious first step is to gather more observations, which are needed for better statistics and finally a reliable determination of the exact heating contribution due to magnetic super tornadoes."