Big northern lights: What's the trigger?

From the Kennedy Space Center in Florida to the Poker Flat Research Range near Fairbanks, Alaska, scientists this week are working to unravel the mystery behind bursts of energetic particles that can turn a so-so display of northern lights into a breathtaking show.

These particles come from the sun and build up within Earth's magnetic field. Then, something snaps and they hurtle toward Earth's poles. The mystery: What triggers the sudden release?

As the particles – electrons – speed toward their shimmering finale, they can wreak havoc on satellites, disrupt radio communications, and trigger blackouts. At high latitudes, these "substorms" can speed corrosion in oil pipelines and befuddle sensors that monitor a pipeline's oil flow and corrosion rates.

For scientists, the long-term goal is to improve forecasts of these geomagnetic substorms to help operators do a better job of reducing the wear and tear on such key communications and energy links.

Friday evening, NASA is set to launch a cluster of five satellites from the Kennedy Space Center that aims to search space near Earth for the triggers of these substorms. The project, dubbed THEMIS, builds on more than a decade of previous space-physics missions. These have helped scientists zero in on substorms as a crucial piece of the space-weather puzzle. Now, the orbiting quintuplets are expected to settle a debate over which of two mechanisms inside Earth's magnetosphere acts as a substorm trigger.

"We call them substorms because they are part of larger space storms," explains Vassilis Angelopoulo, a researcher at the University of California at Berkeley who is the THEMIS mission's lead investigator. The intensity of the overall storm depends on the number of substorms that occur and the conditions surrounding them, he continues. "So substorms are more like a catalyst" for the overall storm's effect on Earth.

The sun is the initial source for charged particles, which billow outward as solar wind. The typical solar breeze can grow to a hurricane, however, when the sun sheds vast amounts of particles in solar flares or larger events known as coronal-mass ejections. If these occur in a region of the sun aimed at Earth, space-weather forecasters sit up and take notice.

The dicey part comes as these particles interact with Earth's magnetic field. The solar wind pushes the sunward side of the field toward Earth and stretches it deep into space on the nightside of the planet, giving it the appearance of a teardrop with a long tail.

Substorms originate on the nightside. But details about where and how – the trigger – have been elusive. One idea holds that electrons flowing in a current along the field hit turbulence and begin to gather in a region of space roughly 50,000 miles above the equator. At some point, the buildup becomes too large and the electrons rocket toward the ionosphere above the poles in a kind of short circuit.

The second idea holds that the trigger takes place 100,000 miles above Earth. The solar wind stretches the magnetosphere's tail to the point where it breaks in two. The "ends" of the portion anchored to Earth reconnect and rebound, releasing energy that hurtles trapped electrons toward the planet.

To settle the trigger question, THEMIS's five satellites will each take up progressively more-elliptical orbits to span the distance these two ideas embrace. Combined with 20 cameras in Canada and other sensing devices on the surface, THEMIS – named for the Greek goddess of justice – is expected to adjudicate the trigger dispute.

"We're really excited about this," says Howard Singer, who heads the science-and-technology infusion branch of the National Oceanic and Atmospheric Administration's Space Environment Center in Boulder, Colo. Satellites that NOAA relies on for space-weather information orbit either very close to Earth or some 23,500 miles out and beyond, he says. Measurements from THEMIS will thus help fill an enormous gap.

But as important as understanding the trigger is, it doesn't tell the whole substorm story, notes Marc Lessard, a space physicist at the University of New Hampshire in Durham. On Monday, he and his colleagues sent a 65-foot sounding rocket arching into the sky above Poker Flat in Alaska to gather images of odd, fainter auroras that pulsate. These pulsating patches of light appear to be associated with a substorm's recovery phase. Most people miss them because once the shimmering sheets vanish, "people pack it in and go home," he says. Pulsating auroras appear to originate in the Van Allen radiation belts, which encircle the planet above the equator.

The amount of energy that particles carry as they head in from the radiation belt is far greater than the energy imparted to particles that generate the brighter aurora. Pulsating aurora can go on for hours, versus 20 to 30 minutes for brighter displays. Thus, they represent an important part of a substorm's energy budget, Dr. Lessard says.

Left to their own devices, particles in the radiation belt would stay there indefinitely, he says. "But we know that they don't stay there indefinitely. They get dumped. We're trying to understand how particles get dumped out of the radiation belts."