The solar system is travelling through much stormier skies than we thought, and might even be about to pop out of the huge gas cloud we have been gliding through for at least 45,000 years. That's the implication of a multi-decade survey of the interstellar wind buffeting the solar system, which has revealed an unexpected change in the wind's direction.
The edge of the solar system is roughly defined by the heliosphere, a giant magnetic bubble blown by charged particles streaming from the sun. This bubble shields Earth from much of the interstellar wind, so a change in the wind's direction will have little effect here on the ground.
But the change does tell us something surprising about our galactic surroundings. The cloud is so large and diffuse, it was previously assumed to be relatively calm, and that the wind would blow in the same direction for millions of years.
"If you took a handful of the air we breathe on Earth and stretched it out to the nearest star, it would have the same density as this cloud," says Priscilla Frisch of the University of Chicago.
The fact that the wind is shifting over the span of mere decades means that the interior of the cloud is either unusually turbulent, or that the solar system is a mere 1000 or so years away from punching its way out.
"Think of looking at clouds in the sky – if you are in that cloud, there's not much change," says Eberhard Moebius of the University of New Hampshire in Durham. "But when you look at the boundary of the cloud, driven around by the wind, you can see fine structures where it's getting fuzzy and pulled around. We think that this might be related to these kinds of turbulent structures."
Solar wind sock
Since the 1970s, we've known that the solar system is moving through a cloud of interstellar gas about 30 light years across, out on the edge of the Milky Way galaxy. The sun's motion through the cloud creates an apparent wind of interstellar particles that slams into the heliosphere.
Most of the wind's particles are charged and so are deflected around the heliosphere by the sun's magnetic field. But some heavier, neutral atoms – mostly helium – make it inside. These helium atoms scatter off the charged particles coming from the sun and create a diffuse glow in extreme ultraviolet wavelengths that is visible across the entire sky.
A US Department of Defence (DoD) satellite called STP 72-1 mapped this glow in 1972 and found that the intensity jumped by a factor of 10 in late November compared with what it had been in June. Around January, it calmed down again. That spike occurred because Earth passed through a build-up of neutral helium atoms as it orbited the sun.
As helium atoms from the interstellar cloud enter the heliosphere, their trajectories are bent by the sun's gravity, creating a cone downwind from the flow of interstellar particles. The cone acts like a wind sock, revealing the direction that the wind is coming from, and it was Earth passing through this cone that provided STP 72-1 with its November spike.
Winds of change
But then NASA's Interstellar Boundary Explorer (IBEX), launched in 2009, revealed something odd: the wind has changed direction. IBEX has been directly sampling neutral helium atoms from the interstellar cloud as part of its mission to map the boundary between the solar system and the rest of the galaxy. Its readings show that, instead of Earth passing through the sun's helium tail in late November, the peak came about a week late, in early December. That indicates a change in wind direction of about 6 degrees in only 40 years.
"We didn't expect any indication of visible changes on the timescale of tens of years," says Moebius. "That's really the surprising thing, in terms of astronomical scales."
To make sure the change was real, Frisch and colleagues gathered historical data from nine other spacecraft, including the original extreme UV measurements from the 1970s, as well as direct helium measurements from the Ulysses spacecraft that flew in the 1990s. They saw a statistically significant trend.
Stuck in the middle
"While there had been hints that something was changing in the environment of the sun, when we finally put all the historical data together it became clear that one can make a strong scientific statement that this change has actually occurred," says Frisch. What the change means is still up for debate. We could be nearing the cloud's edge, or we could still be in the thick of it, pushing our way through an interstellar storm.
"It's possible we're seeing a structure that is not necessarily an edge," says Robert Meier, now at George Mason University in Fairfax, Virginia, who helped make the original STP 72-1 measurements. "A change of direction of flow in a stream could mean you're near the bank, or that there's a rock in the middle of the stream or something like that. It's always harder to figure out what's going on when you're in the middle."
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