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For years scientists have been left baffled as to why the sun's outer atmosphere - its corona - is more than 200 times hotter than its surface. 

But experts believe they have now found the answer after spotting millions of tiny explosions in the sun's atmosphere.

Evidence has revealed that 'nanoflares' are heating the plasma in the corona, causing it to reach temperatures much higher than the surface.

Scientists in Indiana have revealed a new theory for the solar corona mystery. Millions of mini '50-megaton' explosions called nanoflares have been found to be taking place on the sun (shown) every second and heating it, causing its atmosphere to be millions of degrees hotter than its surface

The evidence was presented at the Triennial Earth-Sun Summit (Tess) in Indianapolis, Indiana today.

Four related science studies came together to reveal how the sun's corona is heated to temperatures of more than a million degrees by explosions that are relatively small for the sun, but are large for Earth. 

According to researcher Dr Jim Klimchuk, from Nasa's Goddard Space Flight Center, each one of these explosions has the 'energy of a 50 megaton hydrogen bomb,' the largest ever detonated on Earth.

By comparison to other flares on the sun, they are relatively small, packing just a billionth the energy of a regular solar flare.

But with millions occurring every second they can 'pack a real wallop and heat to high temperatures,' said Dr Klimchuk.

This research provides the first evidence that such nanoflares exist.

Evidence was gathered using a variety of telescopes including Nasa's Solar Dynamics Observatory (SDO) and the NuStar telescope, more commonly used to observe bright black holes in the distant universe.

When a nanoflare goes off, it heats the solar atmosphere to a huge 10 million Kelvin (20 million °F, 11 million °C), and this super-hot plasma is the 'smoking gun' for the solar corona.

The resultant flash of heat causes temperatures to rise dramatically, and as so many are occuring every second, the temperature remains high. 

One mystery still remains, though - the scientists are not quite sure what is causing the nanoflares, but the dominant processes are thought to be the breaking of magnetic fields.

When the twist and tangles become so great and the fields break it causes a nanoflare.

Key evidence comes from sounding rockets, which are small rockets launched on brief trips into the atmosphere, lasting only around 15 minutes in total.

When a nanoflare goes off it heats the solar atmosphere to a huge 10 million Kelvin (20 million °F, 11 million °C), and this super-hot plasma is the 'smoking gun' for the solar corona. Shown is a slide from the research that was presented today, revealing what happens when a nanoflare goes off

The Extreme Ultraviolet Normal Incidence Spectrometer (Eunis) telescope (shown) was used to gather data from the sun. It was launched on a sounding rocket, which flew into space for just 15 minutes but was in orbit long enough to observe the sun and find evidence for nanoflares

Rice University and the University of Glasgow were also involved in the groundbreaking research.

Tess is a joint meeting of the American Astronomical Society's Solar Physics Division and the American Geophysical Union's Space Physics and Aeronomy section.

This was the first in a triennial series of meetings to unite the field of heliophysics - the study of the sun. 

Scientists are seen here recovering the Eunis telescope, which was used to find evidence of nanoflares

Evidence was also gathered using Nasa's Solar Dynamics Observatory (SDO), illustrated, and the NuStar telescope, more commonly used to observe bright black holes in the distant universe

One mystery still remains, though. The scientists are not quite sure what is causing the nanoflares, but the dominant processes are thought to be the breaking of magnetic fields, such as during a regular - and much larger - solar flare, like the one pictured

The temperature of the sun is known to be around 6,000°C (10,800°F), but for some reason the corona is around 1,000,000°C (1,800,000°F), and even reaches 2,000,000°C (3,600,000°F) in some areas.

This factor of almost 200 difference would be akin to the flame of a fire being 200 times colder than the air around it - but why this is occurring was unknown.

A previous theory was that magnetic forces on the sun couple with superheated gas create a strong magnetic field and funnel energy into the corona, but this has not yet been confirmed.

And now new evidence points to miniature explosions occurring in the solar atmosphere, due to the breaking of magnetic fields.

One of the best times to observe the corona is during a solar eclipse on Earth, one shown in 1999, with the moon blocking out the brunt of the sun's light and leaving only the atmosphere visible

The temperature of the sun's surface (shown in this Nasa image) is known to be around 6,000°C (10,800°F), but for some reason the corona is around 1,000,000°C (1,800,000°F), and even reaches 2,000,000°C (3,600,000°F) in some areas, a factor of about 300