Particles flying towards it from interstellar space, by contrast, have jumped markedly in the past year. It all points to an imminent departure, which would make Voyager the first man-made object to cross into the space between the stars.
Dr Stone said, "It's hard to imagine there's another layer between the one we're in and the outside. Topologically, it makes sense that this is the outermost layer. The only question is: how thick is it?"
Launched way back in 1977, the probe has now travelled so far from home that its constant chatter of data takes 17 hours to arrive at the US space agency's receiving network. Voyager's instruments are busy sampling the far-flung environment. This has allowed Dr Stone and colleagues to map the shape and reach of the heliosphere - the giant bubble of charged particles blown off from our Sun.
In 2004, it reached a turbulent region referred to as the heliosheath, where particles bounced around in all directions. It was expected this would be the final stage before the leap to interstellar space. But, as has been the case throughout this 35-year mission, Voyager threw up yet another surprise.
Last year, it detected what appears to be a discrete boundary layer that Ed Stone's team call the "heliosheath depletion region" in Friday's three Science papers.
It is a kind of magnetic highway where energetic particles on the inside can get out easily, and the galactic cosmic ray particles on the outside can zoom in. "It is where the Sun's magnetic field has piled up, compressed up against itself. It has also doubled in strength. It's smoother than anything we've ever seen with Voyager," Dr Stone explained.
The team is now watching the direction of the field lines very carefully. Currently, they orientate east-west, wound into a spiral by the rotating Sun. But when Voyager finally breaks through into interstellar space, they are expected to shift dramatically, running north-south.
Read more about the journey of the Voyager from here.
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