Millions of H-bomb-sized explosions heat sun's corona
Scientists have long puzzled over why the sun's atmosphere is so much hotter - 300 times hotter - than its surface, despite it being further away from the heat source.
Now, researchers believe they have solved the mystery. Briefing reporters at the Triennial Earth-Sun Summit meeting in Indianapolis on Tuesday, researchers said they have identified a mechanism at work that depends on intermittent bursts of explosive heat called nanoflares, contradicting an earlier theory that pointed to continuous gradual heating.
Jim Klimchuk, a solar scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said the new evidence supports a theory that millions of nanoflares heat the sun's corona. These individual bursts of heat can reach temperatures of some 10 million kelvins or 18 million degrees Fahrenheit, providing heat to the sun's atmosphere.
"The explosions are called nanoflares because they have one-billionth the energy of a regular flare," Klimchuk said in a statement. "Despite being tiny by solar standards, each packs the wallop of a 10-megaton hydrogen bomb. Millions of them are going off every second across the Sun, and collectively they heat the corona."
Scientists zeroed in on the nanoflares as far back as 2009, after finding that coronal loops have much higher density than could predicted with the steady heating theory. But they still needed direct evidence the nanoflares existed.
The first evidence of these nanoflares was presented in 2013 by Adrian Daw, a solar scientist at Goddard and principal investigator of the Turning to the Extreme Ultraviolet Normal Incidence Spectrograph, or EUNIS, sounding rocket mission. EUNIS flew on a 15-minute flight in December 2013 equipped with an instrument called a spectrograph. It was tuned into a range of wavelengths useful for spotting material at temperatures of 10 million kelvins (18 million degrees F), the temperatures that signify nanoflares.
The spectrograph spotted this extremely hot material in active regions that visibly appeared to be quiet. In a quiet region, such hot temperatures clearly weren't due to a large explosive solar flare, raising the possibility that the heat source for the sun's atmosphere had been found.
On top of that, Daw said evidence was also found from another experiment launched on sounding rockets in 2012 and 2013 that imaged soft X-rays from the corona. These results, too, confirmed the presence of superhot plasma on the sun.