The first detailed images of the violently turbulent transition region that lies between the sun's visible surface and its corona may give scientists clues into one of the big baffling mysteries of solar physics -- Where is the energy source for the corona's intense heat?
While the visible surface of the sun is somewhere between 20,000 and 50,000 degrees Fahrenheit (11,000 and 28,000 degrees Celsius), the temperature of the corona that surrounds it -- roiling millions of miles into space -- is more than 2 million degrees Fahrenheit.
"The basic question is how does the corona get to be millions of degrees when it's expanding from a 50,000-degree surface essentially into vacuum. That's not at all understood," said Thomas Berger, a physicist at the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, California.
Berger and his colleagues think they are on the track of the answer. It may lie, they say, in newly-observed phenomena called "solar moss" -- features named for a splotchy, sponge-like appearance that resembles moss. Images from the solar-observing Transition Region And Coronal Explorer (TRACE) satellite, have revealed the rough-looking features in the very lowest levels of the corona. TRACE is sensitive to far ultraviolet light, which lies between ultraviolet and X-ray region of the spectrum.
"When we look at the moss, we're looking directly at a region of the solar atmosphere called the transition region," where the temperature jumps from relatively cool temperatures to the million-degree coronal temperatures, Berger said. "Nobody's been able to really image that before and see these flows and dynamics and the various effects that we see in the moss."
Berger and his colleagues, Bart De Pontieu and Lindsay Fletcher, also physicists at Lockheed Martin's Solar and Astrophysics Lab, noticed the splotchy features when TRACE first started sending back images of the sun in 1998. The rapidly changing blotches appeared at the bases of some huge arches of plasma called
In a quest to understand the moss, Berger, Fletcher and De Pontieu spent more than a year analyzing TRACE data and making other observations with various solar-observing instruments, including the Solar and Helospheric Observatory (SOHO) spacecraft and the Swedish Vacuum Solar Telescope on the Spanish island of La Palma. The team presented their results Tuesday at the annual meeting of the American Geophysical Union in San Francisco.
The moss is essentially caused from the turbulent clash of the hot and cool layers of plasma in the transition region, the group announced.
"You basically have this cold gas that looks pretty dark in the ultraviolet image. And you have this hot gas just lying next to it. And that 's why it looks like moss," De Ponteau said in an interview Monday.
Speaking in the same interview, Berger and Fletcher explained that the cool gas from the lower regions seems to shoot up from the chromosphere -- the layer just below the corona.
"It just blows a huge hole in the overlying hot gas as it goes, Fletcher said. "It just pushes it all out of the way." The cold gas, which is dark in the TRACE images, is much more dense than the superheated plasma above, so it has an easy time shoving hot gas out of the way, she said.
"This dark stuff shoots up at velocities of something like 10 miles (16 kilometers) per second (36,000 m.p.h.) or so, upwards and then falls back down," De Pontieu said. The extremely dynamic regions can spread out across vast areas of the sun's surface, traveling as much as 1,800 miles (3,000 kilometers) in just a few minutes. Moss often travels in "fronts," covering areas several times greater than the surface of Earth in tens of minutes.
Although the group has no real guesses about what drives the movement of the moss, or what causes these regions to spread across the sun, Fletcher said that careful observations help physicists determine just how energy is transferred through the sun and into the corona.
"Just being able to look at that must eventually tell us something about the energy source for the whole corona," she said.
TRACE is operated jointly by NASA's Goddard Space Flight Cener, the Lockheed Martin Solar and Astrophysics Lab , the Smithsonian Astrophysical Observatory and Montana State University.
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