An international group of scientists led by the Mullard Space Science Laboratory (MSSL), University College London, has discovered important new evidence that points to the cataclysmic events that trigger a solar flare and the mechanisms that drive its subsequent evolution.

The new results, described at the RAS National Astronomy Meeting in Birmingham by Dr. Louise Harra of MSSL, were obtained by using the ESA-NASA SOHO spacecraft to study an X-7 class flare -one of the most powerful solar flares of recent years- which exploded on the Sun's surface on 15 July 2002.

Detailed analysis shows that the 15 July 2002 flare was a complex event with three eruptions -each one triggering the next one like a domino effect. The solar flare's explosive power was 5,000 million times greater than an atomic bomb, hurling a billion tonnes of hot gas towards the Earth at speeds of around half a million miles an hour.

The SOHO data show that the explosion was triggered by the sudden emergence of a strongly magnetised gas (plasma) from below the Sun's surface, close to an existing region of strong magnetic field. When the magnetic fields in these two regions collided, they released phenomenal amounts of energy.

The data showed that the entrained gas was heated to a temperature of up to 20 million degrees Celsius before it soared into the solar atmosphere - the corona - at a speed of 90,000 mph.

The analysis of this flare contradicts the standard model of how flares are created. Until now, it has been believed that magnetic field lines from the core of the active region become entangled and reconnect high in the corona.

"In the case of the July 2002 flare, magnetic reconnection had to occur over a much wider area than expected," explained Dr. Harra. "The data show that small-scale magnetic disturbances around the main active region snowballed until they broke through the weak field that blanketed the region.

"This magnetic build-up swept away the existing magnetic field lines that overlay the entire active region, so allowing the core field to reconnect. We were able to measure this by looking for flashes of ultraviolet light and fast-flowing gas.

"This is important, because we have observed the flows of hot gas for the first time, enabling us to see that several small flares combine to create a major explosion. This may eventually enable us to predict large flares before they erupt."

This research gives us a tantalising glimpse of the new insights that can be expected when Solar-B, a joint UK-Japan-US space mission to study the Sun, is launched from southern Japan in summer 2006.

"Solar-B will allow us to study in unprecedented detail the forces that create these explosions," said Dr. Harra, the UK project scientist for Solar-B.

"The spacecraft's polar orbit will allow its instruments to stay in continuous contact with the Sun for nine months each year," explained Professor Len Culhane of MSSL, the UK Principal Investigator on Solar-B.

"Onboard are instruments that will for the first time allow us to measure the strength and flow direction of the magnetic field as well as the flows of gas seen in the atmosphere in response to colliding magnetic fields."

These advanced capabilities will ultimately enable scientists to predict the occurrence of flares and other eruptive events on the Sun, such as coronal mass ejections - a capability that will become even more important with the increasing interest in sending humans to Mars.

The Extreme Ultraviolet Imaging Spectrometer Instrument that will carry out these measurements on Solar-B was built in the UK, under the leadership of MSSL. The instrument is now being integrated with the spacecraft in Japan.