Image: NASA Glenn Research Center

31 August 2007—Researchers at Los Alamos National Laboratory, in New Mexico, say they have solved the mystery of satellite-zapping “killer electrons” that are sometimes produced in Earth’s outer atmosphere. These highly energetic electrons—strong enough to damage electronics and human tissue—pose a danger to spacecraft, satellites, and astronauts. For many years, the mechanism by which they are produced has remained little understood, in spite of physicists’ attempts at solving this puzzle.

Now, Yue Chen, Geoffrey Reeves, and Reiner Friedel say they have conclusively proved that killer electrons come about because very-low-frequency electromagnetic waves—themselves of somewhat mysterious origin—accelerate ordinary electrons in the Van Allen radiation belts to a point where they are traveling at velocities close to the speed of light. The three scientists published their results in the July 2007 issue of the journal Nature Physics.

If they are right, it would be a big step toward understanding the physics of the Van Allen belts and could pave the way to space-weather forecasts. Such reports would be valuable to airlines and operators of satellites and telecommunication systems, because storms in the Van Allen belts regularly disrupt communications and occasionally damage satellites.

“Our results are crucial for the development of a predictive model of space weather,” says Chen. And with space-weather forecasts, “actions can be taken to protect satellites.”

The Van Allen belts, named after their discoverer, James Van Allen (1914–2006), are two concentric, roughly doughnut-shaped zones of radiation containing electrons and ions that extend thousands of kilometers into space. They are caused by interactions of the Earth’s magnetosphere with cosmic rays and the solar wind—the stream of charged particles that is continuously emitted by the sun. Geostationary satellites orbit within the outer belt; many low-orbiting satellites encounter the belts only if they pass near the poles, where both belts bend in toward Earth.

The electrons in the Van Allen belts are normally not dangerous to satellites and spacecraft, but every month or so radiation levels spike to as much as a thousand times their usual intensity. These surges—called geomagnetic storms—are related to increased intensity of the solar wind and are associated with spectacular auroral displays over the poles. But until now there was no definitive understanding of how relatively harmless electrons get accelerated to become killer electrons.

“Debates on the source of the acceleration have lasted for at least a decade, and this paper finally settles the argument based on observations,” Chen says.

Part of the reason the problem has been so tough to crack is that some of the electromagnetic wavelengths that need to be studied to understand the electrons’ behavior range in the tens of kilometers—far too long for laboratory experiments. Addressing the problem also requires data from more than one altitude above the Earth’s magnetic equator, where differences in a key term called “electron phase space density”—a measurement of the flow of electrons traveling through an area divided by the square of their momentum—would be the most telling. Getting more than one data point from the magnetic equator was something scientists had not previously done.

Chen and his colleagues managed to work out the logistics by using data from an electron detector aboard a Global Positioning System satellite, and by using other particle detectors aboard a geosynchronous satellite and NASA’s Polar satellite.

Their observations seem to imply that the killer electrons could result only from an acceleration process produced by the interaction of charged particles and electromagnetic waves. However, the experiment did not explain the exact mechanism of the interaction, says Reeves, adding that while they had conclusively proved the process, much work still needed to be done to understand how it works. The source of the electromagnetic waves is not a settled issue. They may be born of turbulence in the flow of electrons within the Van Allen belts themselves or they may emanate from lightning in the Earth’s atmosphere.

In a competing theory of killer electrons called “radial diffusion,” the Earth’s magnetic field lines are thought of as acting like an elastic band. When plucked by a burst of solar wind, the field wobbles and vibrates. If an electron in the Van Allen belt happens to be wobbling at the same rate, it will gain energy and accelerate across the magnetic field. But the pattern of electron phase space density picked up by the three satellites matched what you’d expect if the electrons were driven by interactions with electromagnetic waves, not radial diffusion.

The Los Alamos team’s experiment “certainly represents a major step in an area of research where controversy still exists and has grown, even after some four decades of radiation-belt investigations,” says Louis Lanzerotti, professor of physics at the New Jersey Institute of Technology, in Newark, and an expert in the physics of charged particles in space.

But Lanzerotti says it is still too early to say whether Chen and his colleagues have proved beyond doubt the mechanism behind killer electrons. He says that the definitive answer will probably come when the NASA Radiation Belt Storm Probes mission is launched in 2012.

That mission, involving two spacecraft, was specifically designed to understand the killer-electron problem. Observations will enable the development of empirical models that engineers can use to design better radiation-hardened spacecraft. The models will also help physicists to predict geomagnetic storms in order to alert both astronauts and spacecraft operators to potential hazards.

Michael Schulz, a space physicist at Lockheed Martin Missile and Space Co.’s Advanced Technology Center, in Palo Alto, Calif., agrees that the jury is still out on whether the Los Alamos team has solved the mystery. “What this really means is that radial diffusion is not the whole story,” he says, pointing out that there are other physical processes that need to be studied in detail, too.

But Stanford electrical engineering professor Umran S. Inan is in the Los Alamos team’s corner, having always suspected that electromagnetic waves were the main cause of killer electrons. Inan, along with colleagues in the U.S. Air Force, actually plans to see if they can do something about the high-energy radiation. By 2010, they plan to launch a satellite, dubbed DSX (Demonstration and Science Experiment), that will test Inan’s theory that low-frequency electromagnetic radiation injected into the lower Van Allen belt could cause the high-energy electrons to prematurely rain out into the atmosphere, potentially ending a monthlong geomagnetic storm in a matter of days.