It is a question that has plagued solar physicists for decades: 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?



Various explanations have been put forth, from sound waves or magnetic waves dissipating in the upper solar atmosphere, or corona, to short bursts of energy known as nanoflares erupting 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.



Short-lived fountains of plasma known as spicules, shooting up from the sun's chromosphere, or lower atmosphere, appear to play a role in heating the corona to searing temperatures at millions of degrees Kelvin, researchers have found. 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 are capable of taking detailed images of the sun every several seconds, the kind of quick-time observation needed to identify transient or rapidly changing phenomena.



The spicules last just 100 seconds or so, shooting upward from the chromosphere at speeds of roughly 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 about three minutes. (De Pontieu, who at the time of the interview was soon to board a plane for London, noted that his journey would take a disappointingly un-spiculelike 10 hours.) De Pontieu and his colleagues from Lockheed Martin, the National Center for Atmospheric Research in Boulder, Colo., and the University of Oslo in Norway report their findings in the January 7 issue of Science.



The spicules show up brightly in wavelengths associated with temperatures much cooler than the corona, in the tens of thousands of degrees K. The plasma at those temperatures rises in jets from the chromosphere and then falls back to the surface. But the spicules are also associated in location and time of occurrence with flare-ups in other wavelengths that indicate plasma temperatures of at least 1 million to 2 million degrees K in the corona. Something, then, seems to be heating spicule plasma to exceptional temperatures as the hotter plasma rises toward the corona.



The fountains last just minutes, but the researchers estimate that they occur often enough to potentially account for a good chunk of coronal heat. "There's so much mass being brought up there that you only need a small fraction of it to make it to coronal temperatures to play a significant role," De Pontieu says.



The new work is "extremely interesting," says solar physicist Kenneth Phillips of the Mullard Space Science Laboratory at University College London. "It could provide the long-sought agent for the heating of the solar corona," he adds, "but there have been similar exciting papers in the past on this." The fact that the spicules seem to be associated with heating events that are more than adequate to keep the corona toasty is reason for optimism, Phillips says.



On a fundamental level, it remains unclear just how the spicule plasma is heated to its extreme temperatures, and the mechanism that produces the spicules in the first place is not easily described by existing models. "We don't really know how this works," De Pontieu acknowledges. "If we can figure out how these things form, then we'll have learned a lot about how the solar atmosphere works."



Although spicules certainly 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 continually replenish the corona with 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 calculation for how much hot plasma spicules provide leaves 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."