Physicists find new clue in coronal heating mystery

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Jan 6, 2011

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.

"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.

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.

Colin Stuart is a science writer and astronomer based in London

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Fear rises among Iranian physicists

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Nov 30, 2010

Academics in Iran have been left in a state of critical fear following the murder in Tehran yesterday of nuclear physicist Majid Shahriari and the attempted assassination of another nuclear researcher, Fereydoon Abbasi.

The separate attacks occurred yesterday morning and both were carried out by unidentified assailants on motorbikes who attached explosives to the victims' vehicles as they travelled through the capital. Both scientists were based in the faculty of nuclear engineering at the Shahid Beheshti University in Tehran and they are both said to be key figures in Iran's controversial nuclear programme.

Iranian president, Mahmoud Ahmadinejad, immediately blamed the attacks on foreign enemies saying that "undoubtedly the hand of the Zionist regime and Western governments is involved". As yet, however, no nation or group has claimed responsibility.

"Everyone is shocked. Programmed assassination of scientists is the last thing we could imagine," says Reza Mansouri, a cosmologist at Sharif University in Tehran and a former Iranian deputy science minister. He confirmed that the widely held view in Iran is that this attack was carried out by foreign agents.

Shahriari was a nuclear researcher and his publication record suggests that he specialized in medical applications of nuclear physics, with a particular interest in modelling neutron transport processes. His role in Iran's nuclear programme was confirmed by Akbar Salehi, head of Iran's atomic energy agency, who says that Shahriari was involved in "one of the great projects" of the agency. "Majid Shahriari was one of my students for years and had a good cooperation with the organization," Salehi told the Islamic Republic News Agency.

Fereydoon Abbasi, the scientist who survived, appears to have a larger involvement with Iran's nuclear programme. In 2007 his name was added to the United Nations Security Council's sanctions list for his involvement in "nuclear or ballistic missile activities".

Monday's attacks follow the assassination in January of another Iranian physicist, Masoud Alimohammadi, a quantum mechanics and field theory specialist at Tehran University, who was killed by a remote-controlled bomb attached to the side of a motorcycle. The similarities between the attacks have left Iranian academics in an acute state of fear.

Despite the government's conviction that the murders have been carried out by Israeli or US special services, there is also speculation that the attackers could have belonged to one of a number of organizations within the Middle East. The speculation is fuelled by the continuing secrecy that surrounds Iran's nuclear programme and the increasingly complicated political situation in the Middle East.

Similarly, the motive for assassinating these particular men is not clear, particularly given their seemingly disparate research interests. One thing they do share is that all three men held roles in SESAME – Synchrotron-light for Experimental Science and Applications in the Middle East – a project that aims to create the region's first major international research centre by building a synchrotron light source in Jordon. Shahriari is listed as having been an adviser to the project, while Abbasi is a member of the Iranian advisory committee to SESAME, and Alimohammadi was a member of the council.

SESAME president Chris Llewellyn Smith, who is also a former director-general of CERN, says that he does not remember Shahriari though the official records state that he did attend one council meeting. Llewellyn Smith does, however, recall meeting Alimohammadi but he, likewise, was only able to attend one meeting before he was killed.

Llewellyn Smith is keen to point out that synchrotrons such as SESAME are not in any sense nuclear facilities. "They are accelerators that are designed to produce intense light with wavelengths ranging from the infrared to X-rays, which are used to study matter on scales ranging from biological cells to atoms," he says.

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