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See Inside March 2011

Feeling the Heat

Short-lived fountains of plasma may explain why the sun's outer atmosphere is hotter than its surface



Courtesy of SOHO/EIT consortium/ESA/NASA

It is a question that has hounded solar phys­icists since the 1940s: Why is the outer layer of the sun’s atmosphere, the region farthest from the heat-producing core, hotter than both the lower atmosphere and the sun’s surface?

Experts have put forth various explanations, from sound waves or magnetic waves dissipating in the upper solar atmosphere, or corona, to short bursts of energy known as nanoflares that erupt as tangled magnetic field lines in the corona reconnect. Now observations from a new generation of sun-observing spacecraft are implicating a different mechanism, one that could provide the corona with a significant portion of its heat by continually delivering hot ionized gas, or plasma, to the upper atmosphere.

Spicules, which are short-lived fount­ains of plasma shooting up from the sun’s chromosphere, or lower atmos­phere, seem to play a role in heating the corona to searing temperatures at millions of degrees kelvins, investigators have found. Spicules, whose origins are somewhat mysterious, last just 100 seconds, rising from the chromosphere at speeds of 50 to 100 kilometers per second. As lead study author Bart De Pontieu of the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, Calif., points out, that is fast enough to travel from San Francisco to London in minutes. De Pontieu and his colleagues reported their findings in the journal Science.

The group based its study on observations from NASA’s new Solar Dynamics Observatory, launched in 2010, and the Japanese Hinode spacecraft, which began service in 2006. Both solar observatories can take detailed images of the sun every several seconds, the kind of quick-time observation needed to identify transient or rapidly changing phenomena.

As spicules measuring in the tens of thousands of degrees kelvins rise from the chromosphere, the researchers noticed, patches of the corona above flare up at one million to two million degrees.

The researchers do not yet know what launches the chromospheric plasma at such high speeds nor what heats it to the extreme temperatures it reaches in the corona. But the link between spicules and coronal heating holds promise for closing the books on a 70-year-old mystery, says Kenneth Phillips of University College London.

Although spicules seem to be important phenomena in certain regions of the sun, time will tell whether they deliver enough hot plasma on a global scale to explain the corona’s tremendous heat, says James Klimchuk of the NASA Goddard Space Flight Center in Greenbelt, Md. Klimchuk calls the new observations “very exciting” but notes that his own preliminary calculations indicate that spicules provide only a small share of the hot plasma in the corona, leaving plenty of room for other, more conventional modes of coronal heating.

For his part, De Pontieu sounds a similar note of caution that the long-standing problem of the corona’s temperature has yet to be conclusively resolved. “I think it’s important to point out that we have not solved coronal heating, but we have provided a piece of the puzzle,” he says. “We’ll see down the line whether this proves to be a dominant process or simply a contributor.”

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