In the summer action film The Fantastic Four, intrepid space travelers are exposed to an intense radiation burst that gives them superpowers--from immense strength to invisibility--once they return to Earth. That premise is science fiction, but radiation encircling our planet does affect both astronauts and orbiting satellites. The activity of the two Van Allen radiation belts has now been explained in a report that should help scientists better predict their behavior and reduce the risk to humans and spacecraft.

Charged particles trapped inside our planet's magnetic field make up the Van Allen radiation belts, located almost 20,000 kilometers above Earth. Solar storms increase the activity in these regions, but what causes the variation in the number of charged particles was unclear. Richard B. Horne of the British Antarctic Survey and his colleagues analyzed data collected during two rare space storms in the winter of 2003 in which sections of the Van Allen belts were drained of electrons. "When the radiation belts reformed they did not increase according to a long-held theory of particle acceleration," Horne explains. "By using scientific instruments in Antarctica and on the CLUSTER mission satellites, we showed that very low frequency radio waves caused the particle acceleration and intensified the belts."

Over one to two days, this wave acceleration can increase the intensity of charged particles by more than three orders of magnitude, the scientists write in a report published today in the journal Nature. "The new information will help spacecraft operators and space weather forecasters who must predict when satellites and missions are most at risk from radiation events," Horne notes, "allowing them to take measures to protect instruments and systems from damage, and astronauts from risks to their health."