Based on an instrument that measures charged particles, Voyager entered the magnetic highway on July 28, 2012. The region was in flux for about a month and stabilized on Aug. 25.
Each time Voyager re-entered the highway, the magnetic field strengthened, but its direction remained unchanged. Scientists believe the direction of the magnetic field lines will shift when the probe finally enters interstellar space.
Stone said that another clue that Voyager has reached interstellar space could be the detection of low-energy cosmic rays and a dramatic tapering of the number of solar particles.
In a statement, Stone said he didn't believe Voyager 1 had yet passed into interstellar space because the probe has not yet detected a change in the direction of the magnetic field.
"This is the last critical indicator of reaching interstellar space and that change of direction has not yet been observed," Stone said.
Voyager 1 and a sister spacecraft, Voyager 2, were launched 16 days apart in 1977 for the first flybys of Jupiter, Saturn, Uranus and Neptune.