New Scientist | Astrophile | Stephen Battersby | 2012 Jun 27
Object: Giant plasma tornadoes
Location: Sun surfaceTen thousand gigantic tornadoes are scouring the surface of the sun. Each of these spinning magnetic storms is the size of Europe, and together they may be pumping enough energy into the solar atmosphere to heat it to millions of degrees – a power that leads one scientist to suggest we could mimic these solar tornadoes on Earth in the quest for nuclear fusion power.
The visible surface of the sun has a white-hot temperature of more than 5000 Kelvin, but it is icy cold compared with the outer solar atmosphere, or corona. The thin plasma of the corona reaches temperatures of more than a million Kelvin – which is strange because temperature usually falls when you move away from a source of heat.
A clue to this phenomenon came in 2008, when Sven Wedemeyer-Böhm at the University of Oslo in Norway spotted intriguing swirls on the sun. These spinning patterns appeared in images from the Swedish Solar Telescope on the island of La Palma in the Canary Islands in an instrument called CRISP, which can probe the lowest level of the solar atmosphere, a fairly thin region called the chromosphere.
"At that point we didn't really understand what was behind these swirls," says Wedemeyer-Böhm. Now, as part of an international team, he has used data from NASA's Solar Dynamics Observatory (SDO) and computer simulations to get a more complete picture.
The SDO can see extreme ultraviolet wavelengths that are blocked by Earth's atmosphere, allowing it to look at the part of the corona where the temperature suddenly rises. With simultaneous observations from SDO and La Palma, the team saw swirls stretching from the chromosphere up into the corona.
Simulations show that each swirl is born when convection near the surface of the sun carries some material downwards through narrow channels. Like bathwater draining through a plughole, the material rotates, creating a twisting amalgam of plasma and magnetic fields above. It looks just like a terrestrial tornado, except 1500 kilometres across with winds blowing at tens of thousands of kilometres per hour.
Crucially, at the top of each tornado more ultra-violet light is given off than elsewhere – a sign that the plasma is being heated at that point. Team member Robert Erdélyi at the University of Sheffield, UK, says that energy is probably channelled in the form of magnetic disturbances called torsional Alfvén waves, which race up the twisted field lines of the tornado to be dissipated as heat in the corona.
This is not the only way that heat can get into the corona. Energy is also carried up in little jets called spicules and intermittently dumped by explosive events such as solar flares. However, the team says that tornadoes could form one of the most important heating mechanisms.
If it works on the sun, why not on Earth? "We could steal this technique from nature," says Erdélyi. At the ITER project in Cadarache, France, scientists are trying to generate power from nuclear fusion, which requires heating plasma to many millions of degrees. Erdélyi suggests that if we can better understand coronal heating, similar processes could be used to inject heat into a reactor. Then scaled-down solar tornadoes might bring light and heat to our homes.
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Magnetic tornadoes as energy channels into the solar corona - Sven Wedemeyer-Böhm et al
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