Space scientists have uncovered the cause of the magnetic disruptions that instigate auroras, the atmospheric light displays in the polar regions of the Northern and Southern hemispheres. The finding should help scientists better understand and predict these energetic phenomena, which can wreak havoc on electrical components on the ground and harm satellites and humans in space.
The sun and Earth are locked in an eternal magnetic dance. Our star continuously spews out streams of charged particles, known as the solar wind, while Earth's magnetic field intercepts and deflects most of them. In the process of collecting all that energy, however, the magnetic field stretches and bends, and eventually it snaps, allowing high-energy solar particles to enter Earth's atmosphere. This influx, known as a substorm, is what ignites the auroras, but it creates more than just pretty nighttime skies. Substorms pack enough electromagnetic radiation to endanger astronauts working in Earth orbit, fry sensitive electronics aboard satellites, and damage electrical power grids. For 4 decades, scientists have been trying to determine what starts substorms and how they unfold.
To find the answers, NASA launched five washing-machine-sized satellites in February 2007. The satellites, collectively called THEMIS, for Time History of Events and Macroscale Interactions during Substorms, orbit in a spread-out formation to monitor changes in Earth's magnetic field at various altitudes in response to the solar wind. An international team using THEMIS data, recorded during a substorm on 26 February of this year, reports online today in Science that they have deciphered for the first time how substorms form. The good news is that the process allows up to several minutes' warning of the storms' onset.
During a teleconference today, space scientist and lead author Vassilis Angelopoulos of the University of California, Los Angeles, explained that substorms begin on Earth's night side through a phenomenon called magnetic reconnection. Here, an overload of energy from solar particles causes the planet's stretched magnetic field to rip suddenly and then rejoin. That process acts like a giant elastic band, snapping much of the overload back toward Earth. Scientists have long known about reconnection, Angelopoulos explained, but the new findings show that it "is linked [to auroras] much more closely and directly than we previously thought."
The data from the THEMIS spacecraft "conclusively resolve the question about whether magnetic reconnection initiates substorms, something that has been hotly debated for decades," says physicist Larry Paxton of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. "It's a terrific paper," says Geoff Reeves, a physicist at Los Alamos National Laboratory in New Mexico. "The THEMIS team has presented results that many of us in the field have waited decades for."
Physicist Reiner Friedel, also at Los Alamos, is more cautious. Taking data from one storm and using them to reach a general conclusion might be an overstatement, he says. "It's a fine analysis of one event," Friedel says, "but if there's one thing we've learned about substorms over the past 40 years it's that no two are alike."