Observations from NASA's Solar Dynamics Observatory (SDO) and the Japanese satellite Hinode show that some gas in the giant fountain-like jets in the Sun's atmosphere known as spicules can reach temperatures of millions of degrees. The finding offers a possible new framework for how the Sun's high atmosphere gets hotter than the surface of the Sun.
What makes the high atmosphere, or corona, so hot — over a million degrees, compared to the Sun’s surface of 10,000° Fahrenheit (5,500° Celsius) — remains a poorly understood aspect of the Sun's complicated space weather system. That weather 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 toward better protecting our satellites and power grids.
"The traditional view is that all the heating happens higher up in the corona," said Dean Pesnell from NASA's Goddard Space Flight Center in Greenbelt, Maryland. "The suggestion in this paper is that cool gas is being ejected from the Sun's surface in spicules and getting heated on its way to the corona."
Spicules were first named in the 1940s, but they were hard to study in detail until recently, said Bart De Pontieu from Lockheed Martin's Solar and Astrophysics Laboratory in Palo Alto, California.
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 toward 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," said De Pontieu.
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.
The scientists found that a large fraction of the gas is heated to a hundred thousand degrees, while a small fraction is heated to millions of degrees. Time-lapsed images show that this material spews up into the corona, with most falling back down toward 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, the scientists believe that if even some of that super-hot plasma stays aloft, it would make a contribution to coronal heating.
Jonathan Cirtain, who is the U.S. project scientist for Hinode at NASA's Marshall Space Flight Center, Huntsville, Alabama, said that incorporating such new information helps address an important question that reaches far beyond the Sun. "This breakthrough in our understanding of the mechanisms that 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 said. "This is a fantastic discovery and demonstrates the muscle of the NASA Heliophysics System Observatory, comprised of numerous instruments on multiple observatories."