SAN FRANCISCO--The northern lights delight viewers with shimmering curtains of red and green far overhead. Often, the curtains are parted by strange dark curls or bands, like anti-aurorae that are void of light. Physicists now have their first detailed look at the electrical circuits that drive these eerie "black aurorae," thanks to a quartet of satellites that flew through one of them earlier this year.
The aurora borealis and their Southern Hemisphere counterparts, the aurora australis, arise when charged particles from the sun buffet Earth's magnetic field. Powerful solar outbursts compress the field and force electrons to cascade down into the ionosphere, a sparse layer of the atmosphere hundreds of kilometers high. These electrons plow into air molecules and cast off visible photons--creating the lights--but that's not the end of the story. To complete a vast circuit between the ionosphere and the magnetic field, other electrons must flow back into space. Scientists suspected this return flow caused the black aurorae.
The European Space Agency's Cluster mission has now confirmed that idea, according to data presented here on 10 December at a meeting of the American Geophysical Union. The satellites, four identically equipped craft named Rumba, Salsa, Samba, and Tango, zipped directly above a black aurora on 14 January at a height of 21,600 kilometers. During this fortuitous encounter, the first three satellites measured a threefold surge in the electric current above the aurora within 200 seconds. When the fourth craft arrived on the scene 100 seconds later, the circuit was gone.
That time scale of a few minutes fits well with models of the circuit's behavior, says plasma physicist Goran Marklund of the Royal Institute of Technology in Stockholm, Sweden. During the surge, the electric field carves a hole in the ionosphere by sucking electrons into space. The black aurora corresponds to the narrow zone through which the electrons are expelled. "The Cluster results are the first to show the temporal evolution of these electric field structures," Marklund says. His team also will publish its results in the 13 December issue of Nature.
Cluster's observations provide "the first look at the future of auroral research," says space physicist Patrick Newell of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. He offers one caveat: The currents observed by Cluster were unusually high, so they might not represent typical aurorae. Newell calls for a dedicated multisatellite mission to explore auroral circuits closer to Earth.