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Space + Flight

21
Feb

This “Energy Ribbon” is Helping Astronomers Map the Solar System’s Frontier

The outer fringes of the solar system are a bit like the Wild West—it’s a region of space where the long arm of the sun’s magnetic field weakens and the outside galaxy begins to assert its dominion. We don’t know much about what happens here, or what the interstellar magnetic field is like.

We can see evidence of the power struggle between interstellar space and our solar system in the form of a mysterious “energy ribbon” of particles, which astronomers are using to determine what, exactly, is going on out there. From what we can see, it’s looking like the forces of the galaxy may hold more sway over our corner of space than we thought.

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A visualization of the energy ribbon as seen from NASA's IBEX spacecraft

Scientists only discovered the ribbon in 2009, thanks to NASA’s Interstellar Boundary Explorer (IBEX) spacecraft, which orbits Earth but looks out to the edge of the solar system. The ribbon is made up of particles that come from the heliopause, the boundary where the influence of the sun’s magnetic field bumps up against the interstellar medium.

David McComas, the principal investigator for the IBEX spacecraft, wrote for NOVA Next last April about the heliopause:

When the magnetic fields of the solar wind hit the magnetic fields of the interstellar medium, they do not intermix. The expanding solar wind pushes against the interstellar medium, clearing out a cavity in interstellar space known as the heliosphere. The boundary of that bubble is where the solar wind’s strength exactly matches the pressure of the interstellar medium. We call it the heliopause, and it’s often considered to be the very outer edge of our solar system.

Here, particles entering and exiting the solar system mingle and exchange electrons. This bartering creates rogue particles that have no electric charge, meaning they fly through space without being deflected by any magnetic field.

But IBEX suggested that these neutral particles from the energy ribbon still retained an imprint of loyalty—the direction they travel in is actually aligned to the outside, interstellar magnetic field. Now, scientists are beginning to confirm this picture. A team led by Nathan Schwadron of the University of New Hampshire compared the IBEX measurements to ground-based observations of cosmic rays, another indicator of the interstellar magnetic field.

These mysterious particles are thought to be survivors of ancient stellar explosions and close encounters with distant galactic cores, and they ride into the solar system packing a lot more energy than the neutral ones. They also have an electric charge, which means they are followers of the galaxy’s magnetic field. And when Schwadron’s team compared a map of them to one they derived from the IBEX data, they looked very similar, offering an independent confirmation that the energy ribbon is aligned with the galaxy’s magnetic field. “That was something we were completely lacking,” he told me. It suggests that the galaxy exerts more influence in our solar system’s border regions than previously suspected, warping the entire heliosphere.

There is, however, one well-known spacecraft that has yet to join this consensus: Voyager 1. NASA declared that the craft left the solar system in September 2013, which means it should be measuring the interstellar magnetic field without interference from the sun. But strangely, Voyager 1’s galactic compass is pointing in a much different direction than the one that Schwadron’s team determined from the IBEX data.

Elizabeth Howell, writing for Space.com:

The few studies examining this issue show little consensus. An October paper co-authored by Schwadron in Astrophysical Journal Letters argued that Voyager 1 could be measuring interstellar plasma coming in through magnetic field lines, but may still be in the heliosheath itself. This stands in contrast to findings from NASA and other science groups saying Voyager 1 is definitively in interstellar space.

The researchers noted that Voyager 1 is picking up its information “at a specific time and place”, but IBEX’s data is collected and averaged across vast distances, so that could also lead to discrepancies.

It’s further evidence that Voyager 1 never crossed a hard border when it “left the solar system.” Instead, it’s traversing a very chaotic region of space, where the laws of physics remain, if not suspended, at least poorly understood.