Why Is The Sun's Atmosphere So Hot?

Why Is The Sun's Atmosphere So Hot?

The 2006 launch of the multinational Hinode satellite changed the picture of the Sun for astrophysicists. For two astrophysicists in particular, the resulting imagery offered a voyage of discovery and the thrill of unraveling a long-held solar mystery. 

Earth's atmosphere can obscure the view of unaided ground-based telescopes, but, unimpeded by this problem, the high-resolution telescope flying on Hinode captures images of the Sun in unparalleled detail. 

It is in these new images that Scott McIntosh, Bart De Pontieu, Viggo Hansteen and Karel Schrijver found the first tantalizing clues that led them to a new way of considering why the solar corona is millions of degrees hotter than the Sun's visible surface.

"Among the regions observed by Hinode is the solar chromosphere, the area separating the Sun's surface — the photosphere — from its extended atmosphere, the corona," explained McIntosh, an astrophysicist working at the NSF-funded National Center for Atmospheric Research's High Altitude Observatory.

Intuitively, the Sun's atmosphere should get cooler with distance from the Sun's surface, but reality doesn't match supposition. Using Hinode imagery, De Pontieu, a scientist at Lockheed Martin's Solar and Astrophysics Laboratory, McIntosh, and colleagues discovered in the Hinode imagery a new type of spicule. 

"Classic" Type-I spicules are jets of dense plasma that shoot up from the chromosphere and, more often than not, return along the same path, said McIntosh. The "Type-II" spicules, which McIntosh and De Pontieu have recently dubbed "radices," are hotter, shorter lived and faster moving than their Type-I brethren. 

"In the Hinode imagery," added McIntosh, "the radices appeared to shoot upward and disappear, often moving at speeds in excess of 100 kilometers per second. These jets likely contain plasma that ranges in temperature from 10,000 to several million degrees Celsius, and have a life span of no more than 10 to 100 seconds. While astrophysicists, including NCAR founder, Walter Orr Roberts, have long studied Type I spicules, it is known that the material in them does not reach typical coronal temperatures — about 1 million degrees — eliminating a connection to coronal heating."

But it was only during a 2008 scientific meeting about Hinode — when a colleague discussed seeing a subtle 100-plus kilometer per second upward velocity component in a coronal region with a strong magnetic field — that De Pontieu and McIntosh caught each other's eye, thinking exactly the same thing: were they possibly seeing evidence of radices reaching coronal temperatures?

Together, they searched for the "ideal" Hinode data set, one in which they were able to trace the columns of plasma ejected from the chromosphere into the corona. Upon identifying the data, each approached the task from a different perspective. 

In comparing their results, they realized that the locations of the radices and the upward velocity signatures seen in the corona were the same. They also found that the velocities of the chromospheric jets and those of the coronal events matched extremely well. 

"This evidence indicates that radices may play an important role in supplying and replenishing the hot mass of the solar corona and wind, explaining the temperature differential between corona and photosphere," McIntosh said. "Our calculations indicate that radices can fill the corona with hot plasma even if only one to five percent of the radices reach coronal temperatures.

Not only did this work provide McIntosh, De Pontieu, Schrijver (also of Lockheed Martin's Solar and Astrophysics Laboratory), and Hansteen (of the University of Oslo) the thrill of discovery, and the excitement of tracing their idea to a breath-taking conclusion, their effort has direct implications for climate research on Earth. 

"Understanding solar processes advances our knowledge of Earth-Sun interactions, providing insights on how UV radiation generated by solar storms affects the Earth's upper atmosphere, stratospheric ozone and — potentially — global climate dynamics over both short and longer time scales," McIntosh explained.

One mission that will help advance understanding of radices is NASA's Interface Region Imaging Spectrograph (IRIS, iris.lmsal.com), which will allow scientists to investigate formation of radices at high resolution. A Hinode follow-up mission is also in the works, and the launch of the Solar Dynamics Observatory in early 2010, will provide an additional series of high-resolution coronal images, available every 10 seconds.

Editor's Note: This research was supported by the National Science Foundation (NSF), the federal agency charged with funding basic research and education across all fields of science and engineering.  Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. See the Behind the Scenes Archive.