NASA's THEMIS mission say they have the answer.

Some particularly colourful, dancing auroras, appearing every few hours, coincide with sudden tremors in the magnetic fields around Earth. Although these substorms have been observed for decades, no one was sure exactly how they were created.

A fleet of satellites has pinpointed the sequence of events that lead to magnetic "substorms" near Earth. These are frequent occurrences that cause auroras and may unleash radiation that can damage satellites.

Now, researchers with NASA's THEMIS mission say they have the answer. The substorms begin far out in space, roughly a third of the way to the Moon, where magnetic fields from the Earth are thrown together and reconnect to sling charged particles back toward the planet, they say.
"This is a question that people have been after since the beginning of the Space Age," says THEMIS principal investigator Vassilis Angelopoulos of the University of California, Los Angeles. "The reason it has not been shown up to now is that we didn't have the right satellites at the right positions and the right times."