Using data collected from four satellites, scientists were able to overturn this view.

"We have found that auroras caused by solar winds are very weak and can scarcely be seen with the naked eye. The splendid and colorful auroras we see are actually caused by magnetic substorms," Cao said.

By studying the data collected by the Cluster satellites of European Space Agency (ESA), scientists found that high-speed flows of electrified gas, known as bursty bulk flows (BBFs), are generated by magnetic substorms.

Auroras are caused mostly by energetic BBFs spiraling down Earth's magnetic field lines and colliding with atmospheric atoms at about 100 kilometers altitude.

"When magnetic substorms occur, BBFs strike our atmosphere, making auroras shine," Cao explained.

BBFs contain electrons and other charged particles such as protons coming from the magnetotail, a region of space on the night-side of Earth where the sun's particle winds push Earth's magnetic field into a long tail, said the scientist.

At the tail's center is a denser region known as the plasmasheet. Violent changes of the plasmasheet are known as magnetic substorms. They last up to a couple of hours and somehow hurl electrons and protons earthward, Cao said.

Scientists were able to discover how the process works by using the four Cluster spacecraft, nicknamed Rumba, Samba, Salsa and Tango, launched in 2000.

The four spacecraft, all carrying identical instruments, flew through Earth's magnetosphere, or the region where the planet's magnetic field is dominant.

Cao became one of several Chinese co-researchers of the ESA's Cluster mission in 1996. His paper about magnetic substorms has been published in the authoritative science magazine, Journal of Geophysical Research.

Another key finding is that the average duration of a BBF is longer than previously thought, said Cao.

"The multiple spacecraft data offered by Cluster revealed more about Earth's magnetic environment than data collected by a single spacecraft," Cao said.

Apart from creating beautiful light shows, substorms also excite Earth's ionosphere, disturbing the reception of GPS signals and communications between Earth and orbiting satellites, said the scientist.

Clarifying the origin of Earth's aurora phenomena will help human beings understand weather in space, which is a key factor in ensuring the safety of space probe missions and astronauts, said Pu Zuyin, a professor at the School of Earth and Space Sciences at Beijing University.

According to Cao, the research team's next task is to build an aurora monitoring system, capable of forecasting and analyzing aurora movements.

The colorful auroras regularly light the higher latitudes in the northern and southern hemisphere. In China, people can see it only in the city of Mohe in northeast China's Heilongjiang Province.

But the history of aurora observation in China can be dated back more than 2,000 years.

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by Staff Writers