Scientists have solved a longstanding mystery of why the Sun's outer atmosphere, or corona, is millions of degrees hotter than its surface. The answer lies in fountains of plasma called spicules that shoot up from the Sun's surface.

The research team observed the one-to-one connection between plasma - the electromagnetic gas that surrounds the sun - that is heated to millions of degrees and the spicules that insert this plasma into the corona.

The scientists found that a large fraction of the gas is heated to a hundred thousand degrees, while a small fraction heated to millions of degrees does not immediately return to the surface. Given the large number of spicules on the Sun, and the amount of material in the spicules, the scientists believe that if even some of that super hot plasma stays aloft it would make a contribution to coronal heating.

These observations are a significant step in understanding observed temperatures in the solar corona, says Rich Behnke of the National Science Foundation (NSF). They provide new insight about the energy output of the Sun and other stars.

Researchers hope a NASA mission, the Interface Region Imaging Spectrograph (IRIS) scheduled for launch next year, will reveal more about the spicule heating and launch mechanism.

The new study, published this week in the journal Science, was conducted by scientists from Lockheed Martin's Solar and Astrophysics Laboratory, NCAR, and the University of Oslo. It was supported by NASA and the NSF, NCAR's sponsor.