NASA's Voyager 1 spacecraft has encountered a new region on the outskirts of our solar system that appears to be a magnetic highway for charged particles. Scientists believe that this is the final region Voyager has to cross before reaching interstellar space, or the space between stars.
Scientists call this region the "magnetic highway" because our sun's magnetic field lines are connected to interstellar magnetic field lines. The connection has allowed lower-energy charged particles that originate from inside our heliosphere—the bubble of charged particles the sun blows around itself—to zoom out, and higher-energy particles from outside to stream in.
Before entering this region, the charged particles bounced around in all directions, as if trapped on local roads inside the heliosphere. Thinking the particles might be colliding against the gaseous boundary of the solar system, scientists operating Voyager's low-energy charged particle detector wondered if the spacecraft had reached the last stop before—or even crossed into—interstellar space. Data indicating that the direction of the magnetic field lines has not changed, however, leads the Voyager team to infer that this region is still inside the solar bubble.
The new results were described last month at the American Geophysical Union meeting in San Francisco.
"If we were judging by the charged-particle data alone, I would have thought we were outside the heliosphere," says Stamatios Krimigis, principal investigator of the Low-Energy Charged Particle, or LECP, instrument, which is based at Johns Hopkins' Applied Physics Laboratory. "In fact, our instrument has seen the low-energy particles taking the exit ramp toward interstellar space. But we need to look at what all the instruments are telling us, and only time will tell whether our interpretations about this frontier are correct. One thing is certain: None of the theoretical models predicted any of Voyager's observations over the past 10 years, so there is no guidance on what to expect."
Since December 2004, when Voyager 1 crossed a shockwave known as the Termination Shock, the spacecraft has been exploring the heliosphere's outer layer, called the heliosheath. Here, the stream of charged particles from the sun, known as the solar wind, abruptly slowed down from supersonic speeds and became turbulent. Voyager 1's environment was consistent for about five and a half years, but then the spacecraft detected that the outward speed of the solar wind slowed to zero. The intensity of the magnetic field also began to increase.
"The solar wind measurements speak to the unique abilities of the LECP detector, designed at APL nearly four decades ago," Krimigis says. "Where a device with no moving parts would have been safer—lessening the chance a part would break in space—our team took the risk to include a stepper motor that rotates the instrument 45 degrees every 192 seconds, allowing it to gather data in all directions and pick up something as dynamic as the solar wind. A device designed to work for 500,000 'steps' and four years has been working for 35 years and well past 6 million steps."
In fact, for the past several months, the entire Voyager spacecraft was commanded to rotate periodically by 70 degrees so that the LECP instrument could measure the solar wind flow in the up-down direction, or north-south according to the ecliptic plane on which the planets orbit the sun. In theory, with the flow in the ecliptic plane having dropped to zero, the plasma should have been headed north at Voyager's position. But the measurements, reported Sept. 6 in the journal Nature, showed that the flow was consistent with zero. "This was a real surprise," says LECP co-investigator Rob Decker, of APL, "because most models were expecting the northward speed to be at least as high as 25 kilometers per second."
Around May 14, LECP also measured a sudden 5 percent increase in cosmic rays—high-energy particles coming in from the galaxy—followed by a similar increase on July 28. This second increase was accompanied by a decrease (by a factor of 5) in the low-energy particles, but this lasted for only four days. A few days later the same up-and-down exchange occurred, but on Aug. 25 the instrument recorded an even larger increase in cosmic rays, bringing the total increase since the end of March to about 30 percent.
The intensity of particles that have an even lower energy than the cosmic rays dropped by more than a factor of 1,000 below that observed since Voyager 1 first entered the heliosheath. LECP scientists say they agree with their colleagues that Voyager has entered a new region but perhaps is not yet out of the heliosphere. Decker says that the distribution of lower-energy particles suggests a magnetic field direction of about 110 degrees to the direction pointing away from the sun but in the ecliptic plane, not drastically different from the direction of about 90 degrees inside the heliosphere.
Edward Stone, Voyager project scientist based at the California Institute of Technology, Pasadena, says, "We believe this is the last leg of our journey to interstellar space. Our best guess is that it's likely just a few months up to a couple of years away. The new region isn't what we expected, but we've come to expect the unexpected from Voyager."
Voyager 1 and 2 were launched 16 days apart in 1977 and, between them, visited Jupiter, Saturn, Uranus, and Neptune. Voyager 1 is the most distant man-made object, about 11 billion miles away from the sun. Voyager 2 is about 9 billion miles away from the sun. While Voyager 2 has seen some gradual changes in the charged particles, they are different from those seen by Voyager 1. Scientists do not think Voyager 2 has reached the magnetic freeway.
The Voyager spacecraft were built and are operated by the Jet Propulsion Laboratory, a division of the California Institute of Technology. The LECP instrument was designed and built at the Johns Hopkins University Applied Physics Laboratory with NASA funding. The Voyager missions are part of the NASA Heliophysics System Observatory, sponsored by the Heliophysics Division of the Science Mission Directorate in Washington.