Out at the edge of the solar system, surprises for Voyager 1 (+video)

"We're now at 122, but I hope it's not 177," Stone says – a distance that would rule out taking measurements, since at the craft's current speed of about 3.6 AU a year, it would reach 177 AU two years after the remaining instruments fall silent for lack of power.

Voyager 1 currently is traveling through the heliosheath, a region between the termination shock – a sudden slowing of the solar wind as it feels the effects of the oncoming galactic wind – and the heliopause.

Last year, the team reported that the solar wind's speed outward had virtually reached zero, as expected for the heliosheath. Theory also suggested that these charged particles also should be deflected north and south with greater intensity as Voyager 1 approaches the outside edge of the sun's bubble where the bubble meets the galactic head wind.

Over the past year, Voyager 1 has dutifully been shifting its orientation so the instrument involved – the Low-Energy Charged Particle detector – can take readings in different directions.

In a paper published Thursday in the journal Nature, the team, which includes Dr. Krimigis, reports that it finds no north-south flow. It's unclear whether this means the craft hasn't traveled far enough yet to detect such flows signaling a final transition to interstellar space, or whether theories miss the mark on what happens inside the heliosheath.

Whatever the explanation, Voyager 1 seems to have hit the sun's version of the doldrums – a region where there is turbulence, but the solar wind has no outward flow nor any flow suggesting deflection.

The data the team used was collected through last February.

Since February, however, Voyager 1's instruments have picked up unexpected swings in the relative abundance of relatively low-energy charged particles originating in the sun and trying to escape, and higher-energy particles coming from the cosmos outside the bubble. And the swings have been appearing more frequently and in some cases with increasing intensity.

Where changes in Voyager data occurred over weeks or months, "now they happen on a day-by-day basis," Krimigis says. "We could be in and out of these ups and downs in the data for another month or another three years, for all we know."

The relative abundance of the low-energy insiders trying to get out and the higher energy outsiders trying to get in represent a key indicator of how close Voyager 1 is to breaking free of the sun's bubble, Stone says.

A third benchmark, and change in the orientation of the magnetic field the solar wind carries versus the field the galactic wind carries will be the final clue – although one that will be fiendishly difficult to measure, he adds.

For now, Voyager 1 appears to have entered a transition region with some kind of connection to the environment outside the sun's bubble.

With the sudden appearance and disappearance of particles from the outside, "We don't know whether these are filaments connected to the outside, or whether we're dancing along the edge of a new region which is connected to the outside," he says. "This is all exploration. There is no model that has predicted this kind of detailed variation as we approach the heliosphere."

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