Superhot solar atmosphere heated by millions of tiny explosions: study

Researchers said Tuesday they have gathered strong evidence that could explain why the sun's outer atmosphere, or corona, is much hotter than its surface.

They found millions of tiny explosions called nanoflares are heating the plasma in the corona every second, causing its temperatures to reach several million degrees Celsius, even though the sun's surface is only 5,700 degrees.

That means that the coronal heating mechanism that has puzzled scientists for decades depends on regular, but intermittent explosive bursts of heat, rather than on continuous gradual heating.

The findings were presented on Tuesday at the Triennial Earth- Sun Summit, or TESS, meeting in Indianapolis, Indiana.

"The explosions are called nanoflares because they have one- billionth the energy of a regular flare," Jim Klimchuk, a solar scientist at the U.S. space agency NASA 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, " Klimchuk said.

According to Klimchuk, when a nanoflare goes off, it can reach incredibly hot temperatures of some 10 million degrees, which is greater than the average temperature of the corona, thus providing heat to the atmosphere.

One of the evidence supporting the nanoflare presence came from the NASA Extreme Ultraviolet Normal Incidence Spectrograph, or EUNIS, sounding rocket mission, which flew on a 15-minute flight in December 2013.

The EUNIS spectrograph clearly spotted material at temperatures of 10 million degrees in active regions that visibly appeared to be quiet.

Because there is no large explosive solar flare in a quiet region, such hot temperatures should be a smoking gun that something otherwise unobservable was heating up this area, the researchers said.

Another experiment launched on sounding rockets in 2012 and 2013 that imaged soft X-rays from the corona also confirmed the presence of nanoflares on the sun.

The researchers also used a sophisticated computational model to demonstrate why spotting signs of the nanoflares has been so difficult.

Researchers at Rice University in Texas and the University of Glasgow in Scotland were involved in the research.

The TESS is a joint meeting of the Space Physics and Aeronomy Section of the American Geophysical Union and the Solar Physics Division of the American Astronomical Society.

It will occur every three years and this is the inaugural meeting to unite various research groups that study the sun in a research field known as heliophysics. Endite