The latest research from a team of international astronomers could help to explain the long-standing mystery of why the Sun's outer atmosphere – or corona – is so much hotter than its surroundings.

The corona, the vast gossamer atmosphere of plasma visible from Earth during a total solar eclipse, can notch up temperatures in excess of one million degrees Kelvin (MK). Several rival explanations have jostled to account for why the corona is unexpectedly over 200 times hotter than the visible surface, or photosphere, of the Sun. However, in recent years one theory has charged from the back of the pack to become a frontrunner in the race to solve the mystery: spicules – or fountain-like jets of plasma. These emanate from the chromosphere, a relatively thin layer separating the photosphere and corona.

Previously, the spicule theory was largely discredited due to an absence of correlating phenomena in the corona itself. Then, in 2007, researchers led by Bart De Pontieu at the Lockheed Martin Solar and Astrophysics Laboratory in California, US, found a new breed of spicule, which they dubbed "Type II"; Type II spicules are shorter-lived but faster moving than their Type I cousins. In his latest research, published in Science, De Pontieu and colleagues now believe they have found evidence implicating Type II spicules in the heating of the corona.

Tell-tale signature

"Spicules play a significant role in coronal heating, which doesn't fit any of the current theories. This also suggests that there is significant heating going on in the first few thousands of kilometres [of the corona], which is very different from what people have assumed before," De Pontieu told physicsworld.com. When the spicule jets occur on the solar disc they leave a tell-tale signature in the spectral lines observed in the chromosphere: fast-occurring blue-shifts, known as rapid blue-shift events (RBEs). De Pontieu used data from the Solar Optical Telescope (SOT), aboard the Sun-orbiting Hinode spacecraft, to build up a catalogue of RBEs, which he then compared to coronal data from NASA's Solar Dynamics Observatory.

We haven't completely solved the problem, but we've certainly added a significant new wrinkle to it Scott McIntosh, NCAR

"The high temporal and spatial resolution of this generation of solar observatories allowed us to discover that spicule events in the chromosphere are correlated to brightenings in the corona," De Pontieu explained. The team found that the vast majority of the spicule plasma is only heated to between 0.02–0.1 MK and sinks back down into the chromosphere. However, the key finding is that a small but significant portion of the plasma is heated beyond 1 MK and uplifted into the corona. The researchers found this process to be ubiquitous across the Sun.

However, the search for a definitive answer to the coronal heating mystery isn't over. "We haven't completely solved the problem, but we've certainly added a significant new wrinkle to it," second author Scott McIntosh, at the National Center for Atmospheric Research (NCAR) in Colorado, explained.

Combination of different mechanisms

Lucie Green of the Mullard Space Science Laboratory at University College London, who was not involved in the research, agrees: "My hunch is that the solution to the [coronal heating] problem is a mix of answers, a combination of different mechanisms. We shouldn't be looking for just one 'golden' answer," she said. "However, this research is something new and it will definitely sit alongside the other explanations," she added.

Whatever mechanism, or mix of mechanisms, is responsible for the soaring temperatures of the corona, finding an answer is important. "Heating causes the corona to expand outwards, forming the solar wind. This is ultimately what drives lots of processes throughout the solar system, so it would be great to better understand how that heat is being put into the solar atmosphere," said Green.

In order to pinpoint the exact role of spicules in coronal heating and to understand what drives and heats them in the first place, De Pontieu hopes to exploit an upcoming NASA mission. "Fundamentally we need new instrumentation. The Interface Region Imaging Spectrograph (IRIS), is due to launch in December 2012 and it is really focused on the physics of the region between the solar surface and corona," De Pontieu explained. "That would really help us to follow up this research," he added.

The research is described in Science.