SAN FRANCISCO — While scientists have made significant progress in deciphering the complex physics of the sun, they remain perplexed by the way energy and plasma move through its interface region, the area between the sun’s surface and corona.
The sun’s photosphere is 5,800 degrees Kelvin, which is rather cool in terms of solar physics, said Charles Kankelborg, a Montana State University physics professor. In contrast, the sun’s corona is more than 1 million degrees Kelvin. “There’s something very strange going on here,” Kankelborg said. “It’s like finding a glass of water in the refrigerator that seems perfectly normal. The only abnormal thing is that there are the flames leaping off of it.”
The corona’s heat seems to come from the interface region, an area that includes the chromosphere and transition region. What is happening in that region to produce such intense heat? Scientists hope to begin answering that question with the Interface Region Imaging Spectrograph (IRIS), a NASA Small Explorer mission scheduled for an April 29 launch on an Orbital Sciences Corp. Pegasus XL rocket from Vandenberg Air Force Base in California.
“This interface region is something we don’t understand at all, both because we don’t have the observations and because until recently we haven’t had the theory or the computing power to understand the physics,” said Jeff Newmark, IRIS program scientist at NASA headquarters. “We have now developed some physics-based models and the computing power needed to run those models. Now we need observations.”
Alan Title, IRIS principal investigator atAdvanced Technology Center’s Solar and Astrophysics Laboratory in Palo Alto, Calif., said a mission like IRIS could have been launched a decade ago, but scientists would not have been able to make sense of the data it obtained. “Recent developments in computer technology have allowed the generation of numerical simulations that can be compared with observations to isolate the various physical process that transfer energy and mass from the solar surface to the heliosphere,” Title said in an emailed response to questions. Researchers have devoted about 100 million computing hours to the task of generating those numerical simulations, he added.
The IRIS spacecraft, which was built by Lockheed Martin’s Solar and Astrophysics Laboratory, is designed to carry into sun-synchronous orbit a 20-centimeter ultraviolet telescope provided by the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and an imaging spectrograph built by Lockheed Martin. With those instruments, IRIS will focus on the interface region, providing observations with better spatial and temporal resolution than any previous missions, Newmark said.
The IRIS team also includes Montana State University in Bozeman, where Kankelborg, an IRIS co-principal investigator, led the team designing the IRIS spectrograph. The NASA Ames Research Center in Mountain View, Calif., plans to spearhead IRIS mission operations. Norway’s University of Olso is assisting in numerical modeling. Stanford University is prepared to acquire and archive IRIS data at its Joint Science Operations Center in Palo Alto, which already performs the same task for the Solar Dynamics Observatory, a NASA mission launched in 2010 under the space agency’s Living with a Star program, an initiative to explore the sun’s impact on Earth.
The Solar Dynamics Observatory uses three instruments to provide detailed imagery of the sun’s corona as well as the magnetic field on the sun’s surface, but does not focus on the area in between, Newmark said. Similarly, the U.S. Japanese Hinode mission launched in 2006 provides detailed spectroscopic observations of the corona and the photosphere. “So Hinode and [the Solar Dynamics Observatory] play very well together, they are complementary instruments, but they both have the same gap that IRIS is meant to fill, which is between the surface of the sun and the upper corona,” Newmark said.
Scientists noted that IRIS is designed to fill that gap in a cost-effective manner. In 2009, NASA awarded Small Explorer development contracts for two missions, IRIS and the Gravity and Extreme Magnetism Small Explorer (GEMS), an effort to use an X-ray telescope to observe matter moving into black holes. The cost of each mission was capped at $105 million. In June, NASA announced plans to cancel the GEMS mission due to rising costs. The IRIS mission has remained under the cost cap, mission scientists said.
“There is a lot of value in this type of timely, relatively low-cost mission,” Kankelborg said. “We have high hopes that IRIS will have a significant impact on science.”
By focusing on ultraviolet light, IRIS also is likely to help researchers understand Earth’s ionosphere. “IRIS data will help us look not only at a tremendously interesting scientific problem — what makes a star hot — it also has a practical side,” Newmark said. “It will increase our ability to forecast space weather by understanding some of the inputs into Earth’s upper atmosphere which might affect Earth’s climate and our near-space environment.”