Mystery (Partially) Solved? 'Heat Bombs' Warm Sun's Outer Atmosphere

Corona Light
mage of the sun's corona, taken by NASA's STEREO Ahead spacecraft on June 8, 2010. The solar surface is blocked out in this view. (Image credit: NASA/STEREO)

New observations from a NASA spacecraft could help solve a persistent mystery — why the sun's atmosphere is so much hotter than its surface

While the sun's visible surface is about 10,000 degrees Fahrenheit (5,538 degrees Celsius), its upper atmosphere, known as the corona, has temperatures in the millions of degrees.

NASA's Interface Region Imaging Spectrograph (IRIS) spacecraft looks at the transition region between the sun's surface and the corona. The satellite recently saw evidence of "heat bombs," which occur when magnetic fields cross in the corona and realign, much like the process that causes solar flares.

"Because IRIS can resolve the transition region 10 times better than previous instruments, we were able to see hot material rushing up and down magnetic fields in the low corona," Paola Testa, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics who led the research, said in a NASA statement. "This is compatible with models from the University of Oslo, in which magnetic reconnection sets off heat bombs in the corona."

This isn't the only way that the corona is heated, but it's one of the contributors. Another factor is when plasma waves from the sun smash into the corona, moving energy into the outer atmosphere.

IRIS, which launched in June 2013, could be a step toward understanding coronal heating, which has puzzled astronomers for decades, NASA officials said. One benefit of the observatory is that it looks continuously at the sun, allowing scientists to see quick-moving events such as the heat bombs.

"The problem of coronal heating was first discovered in the 1940s," Bart De Pontieu, a solar physicist at the Lockheed Martin Solar & Astrophysics Laboratory, said in the same statement. "The problem involves a variety of complex physical processes that are difficult to directly measure or capture in theoretical models."

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Elizabeth Howell
Staff Writer, Spaceflight

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: