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Solar flares occasionally erupt from the Sun's surface. A solar flare is a violent solar activity that is related to a sudden burst of electromagnetic waves and a large release of charged particles (mostly electrons) from the Sun. So, what is the physical mechanism behind the eruption?

Solar activities have been closely monitored since the development of advanced solar instruments. Scientists found that solar flares normally erupt near sunspots where comparatively stronger magnetic field is located. They generally agree that the energy released in a flare has been stored in the Sun¡¦s magnetic fields. Lines of magnetic field extend from the surface into the corona displaying arching shapes in broad loops. These loops trap extremely hot gas, where the temperature reaches 10 to 40 million degrees causing confined gas to emit extreme ultraviolet radiation and x-rays. But how does the energy trapped in the magnetic field turn into heat and light?

(Source: TRACE (Transition Region and Coronal Explorer))

The breakthrough in our understanding came in 2002, when NASA launched the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) that captures detailed images of solar flares. Dr. Gordon D Holman, astrophysicist in NASA, revealed in his research that solar flare is the consequence of magnetic reconnection, a phenomenon of sudden rearrangement of magnetic field lines. After examining pictures taken by RHESSI, Dr. Holman and his colleague discovered that "bubbles" of plasma emanated from the Sun at the time when broken magnetic field lines reconfigured themselves. The bright light was actually x-rays coming from overheated plasma and acceleration of electrons into space.

Reference: Scientific American, April 2006