Published online 6 January 2011 | Nature | doi:10.1038/news.2011.1

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Plasma jets key to enduring solar mystery

Why the Sun's corona is hotter than its surface.

Multiwavelength extreme ultraviolet image of the Sun taken by the Solar Dynamics Observatory's Atmospheric Imaging AssemblyThe puzzlingly high temperatures seen above the Sun's surface may be due to jets of plasma emanating from the chromosphere.Bart De Pontieu

It's been a mystery for more than half a century: why, in the short distance from the Sun's surface to its corona, or outer atmosphere, does the temperature leap from a few thousand to a few million degrees? The answer, researchers say, might lie in hot jets of plasma erupting from the Sun's surface1.

"It's truly a breakthrough in the longstanding puzzle of how the corona gets so hot," says Rob Rutten, a solar physics expert at Utrecht University in the Netherlands who was not involved with the work. "The jets behave like bullets shot upwards, causing hot coronal temperature fronts in front of them."

Over the years, theorists have offered various explanations for the hot corona. One idea is that the Sun's violent inner motion shakes its magnetic field lines, sending waves through the atmosphere and into the corona that deposit their energy as heat2. Another posits that the magnetic field lines become so twisted that they snap, accelerating and heating the coronal gas3. However, there has been little observational evidence to support either of these theories.

Plasma jets have also been considered as a possible heating mechanism. These jets are known to travel several hundred kilometres from the 'chromosphere' layer just above the Sun's surface to the corona. Yet in the past, rough observations of plasma jets suggested them to be too cool for coronal heating, with temperatures similar to that of the chromosphere itself — just a few thousand degrees.

In a paper published today in Science1, however, Bart De Pontieu of the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, California, and his colleagues show that most of the plasma making up the jets is heated to hundreds of thousands of degrees on its way to the corona, with a small fraction reaching millions of degrees. On the basis of the jets' frequency and intensity, the researchers estimate that they deliver energy "of the order that is required" for the corona to sustain its high temperature. "We are not saying that this is the only mechanism to heat the corona," says De Pontieu. "Clearly, however, these events deserve more attention."

Jet jury

Key to De Pontieu and colleagues' achievement is the instrumentation that they used — the Japanese Hinode space mission and NASA's Solar Dynamics Observatory (SDO). Hinode showed that the plasma jets are initiated at lower temperatures, whereas the SDO revealed higher temperatures in those same jets when they were on their way to the corona (see video). The SDO also resolved the localized brightening of the corona as it was struck by individual jets (see video).

Other solar physicists agree that the work of De Pontieu's group is a major development, but say that it is not the end of the corona mystery. James Klimchuk of NASA's Goddard Space Flight Center in Greenbelt, Maryland, says that he has performed preliminary calculations showing that the plasma jets actually account for only a small proportion of the coronal heating. "The new observations are very exciting, but the jury is still out on the importance of jets in the big scheme of things," he says.

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Mike Wheatland at the Sydney Institute for Astronomy in Australia also thinks that the mechanism is less than clear-cut. "The estimate of the energy supplied by the observed events is of the order required, as the authors state, but the events are localized and occur low in the corona, and coronal heating is required essentially everywhere," he says.

There is no theory yet to explain how the jets form, or why they become heated. De Pontieu thinks that they are probably rooted in magnetic field disturbances similar to those described by previous theories. "This needs to be focused on, going forward," he says. Future solar missions, such as NASA's Interface Region Imaging Spectrograph (IRIS), which is due for launch next year, may offer clues.

"My view is that a final resolution of the coronal mystery will be achieved when we can reconcile the observations with detailed, physical models," says Harry Warren of the US Naval Research Laboratory in Washington DC. "I think it's premature to consider the case closed." 

  • References

    1. De Pontieu, B. et al. Science 331, 55-58 (2011).
    2. Davila, J. Astrophys. J. 317 514-521 (1987). | Article | ISI
    3. Parker E. N., Geophys. Astrophys. Fluid Dyn. 50 229 (1990) | Article
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