Why is the sun's atmosphere hotter than its surface?

NEW CLUE MAY SOLVE SOLAR MYSTERY

A new look at the sun using views from two different spacecraft has put a big dent in the nagging mystery of why the sun's atmosphere, or corona, is millions of degrees hotter than its surface.

The key, it turns out, are called spicules, which vent hot gases, called plasma, from the sun's surface into the corona.

In years past, spicules had been judged too cool to be a source of the high temperature plasma that, among other things, generates the sun's ultraviolet light. A few years ago, a new kind of faster, shorter-lived spicule was discovered. These Type II spicules shoot up at 100 to 150 kilometers per second (62 to 93 miles per second), then disappear.

That behavior made them rather suspicious, but until now, there was no way to check whether they were at least part of the coronal heating puzzle.

"It's a little jet, then it takes off," said solar physicist Scott McIntosh of the National Center for Atmospheric Research's High Altitude Observatory. Those jets have now been measured sometimes exceeding a million degrees. "What we basically find is that the connection is the heated blobs of plasma. It's kind of a missing link that we've been looking for since the 1960s."

The discovery was made by combining data from the new Atmospheric Imaging Assembly on NASA's recently launched Solar Dynamics Observatory with data from NASA's Focal Plane Package for the Solar Optical Telescope on the Japanese Hinode satellite. The combined observations gave the researchers enough resolution to reveal a one-to-one connection between the heated plasma and spicules jetting out into the corona.

"It's not solving the problem but adding a little bit of clarity," said McIntosh, a coauthor of a paper reporting on the discovery in the latest issue of the journalScience.

This is not an isolated event, said McIntosh. The Type II spicules appear to be doing it everywhere, he said.

"This phenomenon is truly ubiquitous and populates the solar wind," said McIntosh, referring to the wind of hot particles that pours out of the sun and out into space.

"It is very nice work, but it is absolutely not the final story on the origin of hot coronal plasma," said NASA solar physicist James Klimchuk. "Based on some simple calculations I have done, spicules account for only a small fraction of the hot plasma."

Most of the heat in the corona, said Klimchuk, is apparently still released by the snapping of stressed magnetic fields in the corona.