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Cracking the code of the interstellar boundary

Dr Amanda Doyle, Feature writer
Feb 4, 2012, 8:00 UTC

Sen— NASA’s IBEX spacecraft is investigating the boundary between our Solar System and what lies beyond, and new results have just been announced.

The Interstellar Boundary Explorer (IBEX) is doing exactly what the name suggests: exploring the interstellar boundary. But what exactly is this boundary and where is it located?

The wind that blasts forth from the Sun doesn’t go on forever; it eventually collides with the interstellar medium and the interaction between the two creates a boundary between the Solar System and interstellar space known as the heliopause.

The area surrounding the Sun where the solar wind is dominant is known as the heliosphere, and being encapsulated in this bubble vastly diminishes the number of harmful cosmic rays that reach the Earth.

The exact location of the heliopause is hard to pin down. NASA’s Voyager 1 spacecraft, launched in 1977, is currently at the edge of the heliosphere. This is known due to recent measurements indicating that the solar wind has subsided to a gentle breeze. But it is impossible to predict whether it will be months or years before Voyager 1 will cross this boundary, as it can’t be seen.

How do we investigate an invisible boundary? Conventional telescopes can’t take pictures of it, because there is no light being emitted from it. However the Earth-orbiting spacecraft IBEX, which was launched in 2008, can detect the particles that are coming from this region.

IBEX monitors the entire sky to get a global view of the boundary and by recording these particles much can be learnt about the heliopause and the interstellar medium beyond.

IBEX can detect both Energetic Neutral Atoms (ENAs) which emanate from the boundary and interstellar neutral atoms which originate in interstellar space.

“We know that these particles are from outside the Solar System because there are lots of them coming from only one direction and there is nothing in the Solar System there,” says Eric Christian, IBEX Mission Scientist at NASA's Goddard Space Flight Center. “If they were from inside the Solar System, we would see the source orbiting about the Sun.”

The latest results from IBEX, published in a series of papers in the Astrophysical Journal Supplement Series, reveal a wealth of information including some unexpected details.

IBEX has measured the ratio of oxygen to neon from outside the Solar System and found that the composition of the interstellar medium is quite alien compared to the Solar System.

It was found that for every 20 neon atoms outside the Solar System, there are 74 oxygen atoms, however for the same amount of neon within the Solar System, there are 111 oxygen atoms.

The Solar System measurements come from looking at the Sun’s composition via spectra. There are two possible explanations for these results. Either the Solar System formed in an oxygen rich region or that a lot of oxygen is trapped within dust grains floating in interstellar space.

Another discovery made by IBEX is that the interstellar wind is blowing at a much lower speed than was measured by IBEX’s predecessor Ulysses. “There are people looking at the old Ulysses data to see if they can understand why it’s different. It may be an actual change in the interstellar wind, or it may be that the difference is just because the IBEX measurements use a newer and more sensitive instrument,” Christian tells Sen.

IBEX has measured the flow of hydrogen, oxygen, and neon from outside our Solar System for the first time. It has also collected data about the amount of interstellar helium, as did Ulysses. Knowing the amount of these elements in interstellar space allows astronomers to map the Milky Way’s evolution through time. This is done by comparing these amounts to those that are observed in stars. “Stars, for the most part, are millions to billions of years old,” explains Christian. “The IBEX measurements give us a measure of the interstellar medium now, at least in this neighbourhood.  So the differences tell us something about how the composition evolved since the Sun (and other stars) formed.”

The speed and direction of the interstellar wind are measured by IBEX by watching how particles are deflected by the Sun’s gravity, as slower particles are deflected more than fast ones. By comparing this speed to that of neighbouring interstellar clouds, astronomers have been able to determine that the Solar System is currently within the Local Interstellar Cloud.

However, this is for a limited time only as evidence suggests that the Solar System is close to the edge of the cloud and will leave it in a few thousand years. When this happens, the heliosphere will expand as there will be less material pushing against it from the outside.

So what might IBEX discover in the future? “All of the new observations from IBEX have surprised us, so this is a difficult question to answer,” says Christian. “My own research recently has been on the first IBEX results, the discovery of a ‘ribbon’ of ENAs. A couple of years after we discovered the ribbon, we still have no idea of what causes it. There are a lot of theories, but I don’t think any of them work. IBEX has generated a lot of puzzles, and I’m hoping new data starts to solve them.”

IBEX will continue to monitor the edge of the Solar System, in the hopes of answering some of the key questions about this mysterious region.

IBEX not only studies the heliopause, but also the termination shock. This is the area where the solar wind first begins to interact with the interstellar medium, and both Voyager craft have crossed it.

IBEX will investigate the strength of this shock, along with how particles are accelerated in this region. It will also monitor the solar wind to determine its properties beyond the termination shock.