The shimmering displays of discrete auroras visible to the naked eye, such as the Northern Lights, delight skywatchers. Scientists, however, are puzzled by their formation. Research suggests that complex circuitry, in which high-energy electrons are accelerated into the atmosphere by electric fields aligned along the direction of the earth's magnetic field, causes these celestial exhibitions. Now a report that will appear in the December 13 issue of the journal Nature provides insight into the creation of these electric fields.

Previous satellite observations of auroral forms were solo missions, which made reconstructing changes to the electric fields over time difficult. But last January the European Space Agency's Cluster satellite formation, composed of four identical satellites arranged like pearls on a string, crossed through auroral field lines 22,000 kilometers above the earth. Goran Marklund of the Royal Institute of Technology in Stockholm, Sweden, and his colleagues analyzed data from the satellitesnamed Rumba, Salsa, Samba and Tangoand determined that the electric fields evolved over a timescale of 200 seconds.

The findings, Patrick T. Newell of Johns Hopkins University writes in a commentary accompanying the report, "support the interpretation that intense aurorae are associated with a reduction of the background electrons in the ionosphere to near-zero densities." This may help explain why the displays tend to form in darkness rather than sunlight, he notes, because sunlight creates free electrons in the ionosphere, the outer region of the earth's atmosphere.