Voyager 2 Finds Edge of Solar System more Complex than Predicted

After more than 30 years in space, NASA's Voyager 2 continues to make new discoveries and is upending our understanding of the processes at work at the very edges of the sun's influence. The spacecraft has found that exotic particles from outside the solar system dominate the dynamics of this distant region, and that it is far more complex than had been predicted.

On August 31, 2007 when the spacecraft reached the termination shock—where the solar wind slows to subsonic speeds as it collides with interstellar gas—it found temperatures lower than what theories had predicted, and it found high-velocity particles that no models had foreseen.

"This was totally unexpected," says Stamatios (Tom) Krimigis of The Johns Hopkins University Applied Physics Laboratory and principal investigator for Voyager 2's Low-Energy Charged Particles (LECP) instrument, which detected the very highest energy particles. "The environment is totally unlike what the models predicted." Researchers from APL and their colleagues are using the new data to modify existing models and create new ones to describe this more complicated reality.

Voyager 2 is traveling toward the outer limits of the heliosphere, a bubble in space created by the solar wind flowing from the sun in all directions. Material from the sun and the interstellar gas outside the heliosphere begin to affect one another at the termination shock, which is more than 83 times farther than the distance between Earth and the sun.

The sudden slow-down of the solar wind at this interface, from approximately 217 miles (350 kilometers) per second to about 81 miles (130 kilometers) per second, releases a tremendous amount of energy. Theories predicted the energy went into heating the now relatively low-speed ions and electrons that make up the solar wind.

"When Voyager 2 measured the temperature of the plasma at the termination shock, there was a mystery," says APL's Rob Decker, lead author on a paper about the mystery in the July 3 issue of Nature. "The temperature was too low to account for the energy loss. We had to ask, where was that energy going?"

It now appears most of that energy is going into accelerating particles from outside our solar system that have made their way into the heliosphere.

While the solar wind races away from the sun at many hundreds of kilometers per second, a relatively light breeze of neutral hydrogen atoms floats into the heliosphere at about 16 miles (25 kilometers) per second. The neutral particles are unaffected by the heliosphere's magnetic and electric fields, but sometimes they collide with solar wind particles and lose an electron, becoming ions.

As freshly charged particles, they are now swept along by the solar wind's magnetic and electric fields, back toward the termination shock. As the rest of the solar wind slows down, the kinetic energy it loses does provide some heating of the solar wind, but most of the energy goes into accelerating the "pick-up" ions, so-called because they were picked up by the solar wind.

Beyond the termination shock is the heliosheath, where the slowed solar wind is diverted away from the approaching interstellar medium and forms our bullet-shaped heliosphere. It now appears that the high-velocity, or non-thermal pick-up ions play a large role in determining the behavior of this region. The heliosheath plasma interacts with and possibly mixes with the interstellar medium across the heliopause, which is the outer boundary of the heliosphere, and the next goal for the Voyager spacecraft.

"Once they are beyond the termination shock, the pick-up ions affect how that medium behaves," says Decker. "They are carrying a lot of energy and therefore play a large role in the dynamics of the flowing plasma, modifying the heliosheath's width and its magnetic field structure, for example. But that's something we're still trying to understand."

The researchers also report in the Nature article on the differences between what Voyager 1 and Voyager 2 have observed at the termination shock. The two spacecraft crossed the interface approximately 10 billion miles apart, Voyager 1 above the plane of the ecliptic—the plane on which most planets orbit the sun—and Voyager 2 below it.

It was expected the two would find themselves in similar environments once across the termination shock, however, Voyager 2 is finding much more variability than did Voyager 1.

"Now it's time to rewrite the models," says Krimigis.

The Voyagers were built by NASA's Jet Propulsion Laboratory in Pasadena, Calif., which continues to operate both spacecraft.