Plasma Science Instrument Finds Surprises At Solar System's Edge

The unexpected findings emerged in the last few weeks as the spacecraft traversed the termination shockwave formed when the flow of particles constantly streaming out from the sun--the solar wind--slams into the surrounding thin gas that fills the space between stars.

The first surprise is that there is an unexpectedly strong magnetic field in that surrounding interstellar region, generated by currents in that incredibly tenuous gas. This magnetic field is squashing the bubble of outflowing gas from the sun, distorting it from the uniform spherical shape space physicists had expected to find.

A second surprise also emerged from Voyager 2's passage through the solar system's outer edge: Just outside that boundary the temperature, although hotter than inside, was ten times cooler than expected. Theorists had to scramble to come up with an explanation for the unanticipated chilling effect.

"It's a different kind of shockwave than we've seen anywhere else," says John Richardson, principal investigator for the Plasma Physics instrument and a Principal Research Scientist at MIT's Kavli Institute for Astrophysics and Space Science. The unexpected coolness, theorists now think, is caused by energy going into particles that are hotter than those that can be measured by the MIT plasma instrument.

Richardson will be taking part in a press conference reporting the new findings on Monday, Dec. 10, at a meeting of the American Geophysical Union in San Francisco.

The Voyager 1 and 2 spacecraft were designed primarily to study the planets Jupiter and Saturn and their moons. After launch, Voyager 2's path was adjusted to take it past Uranus and Neptune as well. Although the craft were only built for a five-year mission, both are still working well three decades later.

"We were incredibly lucky to have it last 30 years," says John Belcher, professor of physics at MIT and former principal investigator for the Voyager Plasma Science instrument. The craft is now expected to keep working until about 2020, and still has important scientific objectives ahead.

It is now passing through a boundary zone called the heliosheath, a region where the solar wind interacts with the surrounding interstellar medium. But sometime in the next decade, it will cross a final edge, called the heliopause, where the sun's outflow of particles ends. At that point, it will be able to measure characteristics of the interstellar medium, for the first time, in a region unaffected by the solar wind and the sun's magnetism.

Although Voyager 1 had already crossed the termination shockwave three years ago, the MIT Plasma Science instrument on that spacecraft had stopped working, so the spacecraft could only indirectly detect the end of the sun's influence.

But with Voyager 2, the Plasma Science instrument not only detected the boundary, making detailed measurements of the solar wind's temperature, speed and density as the spacecraft crossed through it, but it actually encountered the shockwave repeatedly. Because the outflow of the solar wind varies with changes in the sun's activity level, building up during large solar flares and quieting during lulls in sunspot activity, the boundary itself pulsates in and out. These pulsations can wash across the craft multiple times, just as a boat landing onshore may cross the ocean's edge multiple times as waves crash in and then recede.

While Voyager 1 apparently made a single crossing, Voyager 2 apparently crossed the boundary five times, producing a wealth of new data. It's even possible that if there are large variations in that solar outflow, the shock layer "could push past Voyager again," says Richardson. "That would give us some idea of how elastic the shock is" -- that is, how far out these pulsations may stretch. Until and unless such detections are made, "we only have models" of how great such variations might be, he says.

Voyager 2 is now 7.879 billion miles from Earth, traveling away at almost 35,000 miles per hour. Voyager 1 is 9.797 billion miles away, going more than 38,000 mph.

The Plasma Science instrument was developed by the late Professor Herbert Bridge and Alan Lazarus, a senior research scientist in the Department of Physics and MIT's Kavli Institute for Astrophysics and Space Science. NASA has sponsored the work.