Something from Nothing
Billions of miles beyond the orbit of Pluto, in the so-called interstellar boundary region where the Sun’s territory gradually gives way to interstellar space, there’s a whole lot of surprising physics going on.
Data taken by the Los Alamos–built particle imager aboard NASA’s IBEX (Interstellar Boundary Experiment) satellite revealed an unexpected “ridge” of atoms running through most of the boundary region. With no explanation for why this band of matter exists, mission scientists are as perplexed about it as they are excited.
“This band was not predicted by any of our models or theories—not even hinted at,” says the Laboratory’s Herbert Funsten, leader of the collaboration that designed, built, and tested the imager. “It speaks to how much more there is to learn about our own little corner of space.”
To their credit, scientists have learned a tremendous amount about the galactic neighborhood. They know that the space between the nearby stars, the local interstellar medium (LISM), is filled with protons and neutral hydrogen atoms in the form of a moderately hot (6000°K) “cloud” some 30 light-years across. The LISM also sports cosmic rays, magnetic fields, and a smidgen of heavy elements, molecules, and dust. But with a density of only 0.1 atom per cubic centimeter, the LISM is rightly said to be filled with next to nothing.
Our Sun and solar system are cruising through the LISM at a respectable 10 miles per second. However, the so-called solar wind, composed of highly energetic hydrogen ions that race outwards from the Sun in all directions at well over a million miles an hour, pushes the LISM out of the way. Thus the wind creates an immense bubble within the LISM known as the heliosphere. Aside from delineating the Sun’s domain, the heliosphere helps protect Earth and the rest of the solar system from dangerous cosmic rays.
The heliosphere has structure, however. As the solar wind spreads outward, it becomes less dense, and at about 9 billion miles from the Sun, its density becomes roughly equal to the LISM’s. At that distant boundary, called the termination shock, fastmoving solar wind ions occasionally collide with LISM particles, with the net result that the solar wind slows down and heats up as it transitions from a supersonic gale into a subsonic breeze. Beyond the termination shock lies a thick interaction region, perhaps 3–4 billion miles across, where the ions finally come to rest—the edge of the heliosphere.
Scientists knew very little about the termination shock or the interaction region. After all, no light or radiation emanates from either location. Yet scientists were able to gather data from and map those regions by cleverly exploiting a neat piece of physics. During a collision, an energetic hydrogen ion can steal the electron from a slow LISM hydrogen atom and become an energetic neutral atom—an ENA. The neutral atoms can be detected by one of two particle imagers aboard IBEX: IBEX-Hi, built by Funsten’s team, and IBEX-Lo. The two detect high- or low-energy ENAs, respectively.
“The ENAs go in whatever direction the ions were heading the instant before the collision,” explains Funsten. “As ions, they were forced to spiral around and follow the Sun’s magnetic field lines, and if you trace out an ion’s trajectory after it is heated at the termination shock, you’ll see that it frequently points back to Earth. A tiny fraction of ions becomes neutral at just the right instant to travel straight into our imager.”
As IBEX circles Earth in a highly elliptical orbit that extends nearly to the moon, the two imagers get a direct view of a small slice of the heliosphere. Each records the number of ENAs with a specific energy that come from that slice. Then as Earth orbits the Sun, more of the heliosphere gets sampled, allowing mission scientists to construct a contour map showing the source of the ENAs.
As summarized in one of five papers recently published online by Science magazine, IBEX-Hi recorded a factor of 2–3 times more ENAs than expected coming from a circular band that runs most of the way around the heliosphere but that is not centered about the Sun’s direction of travel. Many believe the galaxy’s magnetic field somehow shifts the ENA distribution, but much work awaits scientists as they try to understand how a band of something emerges from nothing.
— Jay Schecker
The IBEX-Hi ENA imager in the Los Alamos calibration chamber, with instrument manager Arthur Guthrie (left) and IBEX scientist Paul Janzen, formerly of Los Alamos and now at the University of Montana.
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