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American Association for the Advancement of Science

Space weather mystery solved?

Artist's rendition of magnetic reconnection triggering substorm onset, as captured by NASA/THEMIS spacecraft.

Near the north and south pole, you can sometimes see colorful light displays in the sky, called the aurora borealis in the northern hemisphere (also known as the northern lights) and the aurora australis in the southern hemisphere.

The auroras are caused by collisions of charged particles in the atmosphere.

Sometimes, explosions of energy in the outer atmosphere cause these "lights" to flare, and scientists have been wondering what causes these explosions. Now they may have an answer.

The Earth is protected from the solar wind by its magnetic field, which surrounds the planet like a large bubble. The protected space inside is called the magnetosphere. It isn't actually round like a bubble; it has a long tail that streams away in the opposite direction from the sun.

In a study appearing in the 25 July issue of the journal Science, Vassilis Angelopoulos of the University of California, Los Angeles and his colleagues analyzed the huge release of energy that spread through the magnetosphere and caused a flare-up of the northern lights last February. They found that the process began all the way out in the magnetosphere's tail.

The magnetic activity that triggered the flare-up occurred very far way the distance down the magnetotail was 10 to 15 times as long as Earth's diameter but the effects spread incredibly quickly. It took less than two minutes for the energy to reach the northern lights, the researchers report.