for National Geographic News
For decades, sudden brightenings of auroras—also called the northern and southern lights—have puzzled scientists.
Such events have been linked to so-called geomagnetic substorms, disturbances in the outer atmosphere that create "power surges" in the polar lights. But what triggers these substorms has remained a mystery.
The research not only could help scientists predict dramatic sky shows, but also should prove a boon for the many technologies—from spacecraft to power grids—that are disrupted by substorms.
Now new data show that powerful explosions in the "tail" of magnetic field lines streaming away from Earth release energy that dramatically brightens auroras.
"What we've just learned is what makes the substorms go off, what triggers the energy release [in Earth's magnetosphere]," said lead study author Vassilis Angelopoulos of the University of California, Los Angeles.
"That's what people have been looking for for the last 50 years."
What Turns on the Lights?
Earth's magnetic field surrounds the planet with a bubble-like envelope called the magnetosphere, which is defined by magnetic field lines that loop between the Poles.
Streams of charged particles from the sun, known as solar winds, rush toward Earth at about a million miles (1.6 million kilometers) an hour. The particles push against Earth's magnetic field lines and cause some of the loops on the side of the planet opposite the sun to stretch outward like a tail.
Auroras occur as solar particles build up and flow along Earth's field lines. These particles become energized as they near the planet, then collide with oxygen and nitrogen atoms in the upper atmosphere, releasing their energy in the form of vibrant bands of red, green, and blue light. (See photos of auroras.)
Occasionally the magnetic field lines release a burst of trapped energy, causing substorms that intensify auroras. Scientists have long debated two leading theories to explain what triggers the storms.
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