Among the many constantly moving, appearing, disappearing and generally explosive events in the sun's atmosphere, there exist giant plumes of gas -- as wide as a state and as long as Earth -- that zoom up from the sun's surface at 150,000 miles per hour. Known as spicules, these are one of several phenomena known to transfer energy and heat throughout the sun's magnetic atmosphere, or corona.
Thanks to NASA's Solar Dynamics Observatory (SDO) and the Japanese satellite Hinode, these spicules have recently been imaged and measured better than ever before, showing them to contain hotter gas than previously observed. Thus, they may perhaps play a key role in helping to heat the sun's corona to a staggering million degrees or more. (A number made more surprising since the sun's surface itself is only about 10,000 degrees Fahrenheit.)
Just what makes the corona so hot is a poorly understood aspect of the sun's complicated space weather system. That system can reach Earth, causing auroral lights and, if strong enough, disrupting Earth's communications and power systems. Understanding such phenomena, therefore, is an important step towards better protecting our satellites and power grids.
"The traditional view is that all heating happens higher up in the corona," says solar physicist Dean Pesnell, SDO's project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. "The suggestion in this paper is that cool gas is ejected from the sun's surface in spicules and gets heated on its way to the corona. This doesn't mean the old view has been completely overturned, but this is a strong suggestion that part of the spicule material gets heated to very high temperatures and provides some coronal heating."
Spicules were first named in the 1940s, but were hard to study in detail until recently, says Bart De Pontieu of Lockheed Martin's Solar and Astrophysics Laboratory, Palo Alto, Calif. whose work on this subject appears in the January 7, 2011 issue of Science magazine.
In visible light, spicules can be seen to send large masses of so-called plasma – the electromagnetic gas that surrounds the sun -- up through the lower solar atmosphere or photosphere. The amount of material sent up is stunning, some 100 times as much as streams away from the sun in the solar wind towards the edges of the solar system. But nobody knew if they contained hot gas.
"Heating of spicules to the necessary hot temperatures has never been observed, so their role in coronal heating had been dismissed as unlikely," says De Pontieu.
Now, De Pontieu's team -- which included researchers at Lockheed Martin, the High Altitude Observatory of the National Center for Atmospheric Research (NCAR) in Colorado and the University of Oslo, Norway -- was able to combine images from SDO and Hinode to produce a more complete picture of the gas inside these gigantic fountains.
Tracking the movement and temperature of spicules relies on successfully identifying the same phenomenon in all the images. One complication comes from the fact that different instruments "see" gas at different temperatures. Pictures from Hinode in the visible light range, for example, show only cool gas, while pictures that record UV light show gas that is up to several million degrees.
To show that the previously known cool gas in a spicule lies side by side to some very hot gas requires showing that the hot and cold gas in separate images are located in the same space. Each spacecraft offered specific advantages to help confirm that one was seeing the same event in multiple images.
[image-69]First, Hinode: In 2009, scientists used observations from Hinode and telescopes on Earth to, for the first time, identify a spicule when looking at it head-on. (Imagine how tough it is, looking from over 90 million miles away, to determine that you're looking at a fountain when you only have a top-down view instead of a side view.) The top-down view of a spicule ensures an image with less extraneous solar material between the camera and the fountain, thus increasing confidence that any observations of hotter gas are indeed part of the spicule itself.
The second aid to tracking a single spicule is SDO's ability to capture an image of the sun every 12 seconds. "You can track things from one image to the next and know you're looking at the same thing in a different spot," says Pesnell. "If you had an image only every 12 minutes, then you couldn't be sure that what you're looking at is the same event, since you didn't watch its whole history."
Bringing these tools together, scientists could compare simultaneous images in SDO and Hinode to create a much more complete image of spicules. They found that much of the gas is heated to a hundred thousand degrees, while a small fraction of the gas is heated to millions of degrees. Time-lapsed images show that this hot material spews high up into the corona, with much of it falling back down towards the surface of the sun. However, the small fraction of the gas that is 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, if even some of that super hot plasma stays aloft it would make a fair contribution to coronal heating," says Scott McIntosh from NCAR, who is part of the research team.
Of course, De Pontieu cautions that this does not yet solve the coronal heating mystery. The main result, he says, is that they're challenging theorists to incorporate the possibility that some coronal heating occurs at lower heights in the solar atmosphere. His next step is to help figure out how much of a role spicules play by studying how spicules form, how they move so quickly, how they get heated to such high temperatures in a short time, and how much mass stays up in the corona.
Astrophysicist Jonathan Cirtain, who is the U.S. project scientist for Hinode at NASA's Marshall Space Flight Center, Huntsville, Ala. points out that incorporating such new information helps address an important question that reaches far beyond the sun. "This breakthrough in our understanding of the mechanisms which transfer energy from the solar photosphere to the corona addresses one of the most compelling questions in stellar astrophysics: How is the atmosphere of a star heated?" he says. "This is a fantastic discovery, and demonstrates the muscle of the NASA Heliophysics System Observatory, comprised of numerous instruments on multiple observatories."
Hinode is the second mission in NASA's Solar Terrestrial Probes program, the goal of which is to improve understanding of fundamental solar and space physics processes. The mission is led by the Japan Aerospace Exploration Agency (JAXA) and the National Astronomical Observatory of Japan (NAOJ). The collaborative mission includes the U.S., the United Kingdom, Norway and Europe. NASA Marshall manages Hinode U.S. science operations and oversaw development of the scientific instrumentation provided for the mission by NASA, academia and industry. The Lockheed Martin Advanced Technology Center is the lead U.S. investigator for the Solar Optical Telescope on Hinode.
SDO is the first mission in a NASA science program called Living With a Star, the goal of which is to develop the scientific understanding necessary to address those aspects of the sun-Earth system that directly affect our lives and society. NASA Goddard built, operates, and manages the SDO spacecraft for NASA's Science Mission Directorate in Washington.