IBEX, The Energetic Neutral Atom Hunter

The termination shock, the point at which the interstellar medium interacts with the heliosphere (NASA)
The termination shock, the point at which the interstellar medium interacts with the heliosphere (NASA)

The Interstellar Boundary Explorer (IBEX) is set for launch in October and it will carry out some pretty cool science when in space. Orbiting Earth 200,000 miles away, IBEX will be on the lookout for tiny, but significant particles originating from the edge of our solar system. Why? It turns out these neutral atoms will provide information on the location and characteristics of the point at which the Solar System’s edge mixes with the tenuous gas between the stars. So how are they generated? How do we know they originate from the termination shock?

On writing an article for the Universe Today earlier (IBEX Mission Will View the Final Frontier of the Solar System), I was intrigued about the physics behind the souce of energetic neutral atoms. Briefly, the theory goes a little like this:

The influence of the solar wind extends to about 94 AU (as measured in-situ by the Voyager 1 probe). At this point, the solar wind slows down as it interacts with the interstellar medium (the tenuous gas occupying the space between the stars). There are a mix of charged ions and neutral atoms. The heliosphere is filled with the solar magnetic field, so as the Solar System travels through the interstellar medium (ISM), the interstellar ions are pushed aside and deflected from entry into the Solar System. Neutral particles are unaffected by the magnetic field however, so they penetrate deep into the heliosphere. The point at which solar wind ions and interstellar neutral atoms interact is called the termination shock. This shock is basically a compression region filled with particles and a turbulent magnetic field.

The physics behind charge exchange where a fast-moving proton captures an electron from theslow moving atom. Source: astronauticsnow.com
The physics behind charge exchange where a fast-moving proton captures an electron from theslow moving atom. Source: astronauticsnow.com

At this interaction region, a phenomenon called charge exchange occurs. The charged protons spiralling from the Sun along magnetic fieldlines embedded in the solar wind strip the outer electrons from the slow moving interstellar atoms. As the electrons pair up with protons, neutral hydrogen atoms are created, otherwise known as energetic neutral atoms (ENAs). Now these neutral atoms are fast-moving by nature and as they have no charge, they fire away from the point of charge exchange in a straight line (as they are no longer influenced by magnetic fields). Some of these fast moving atoms are directed toward the Earth and can be detected by instrumentation orbiting our planet. (The instrumentation must be located outside the Earth’s magnetosphere as the geomagnetic field also produces these ENAs.)

IBEX is one such instrument monitoring the ENAs being generated at the termination shock of the Solar System. These ENAs can be detected and point of generation mapped. Therefore we have a remote means of understanding the nature of the termination shock where these neutral atoms are being abundantly created.

For more information about NASA’s IBEX mission, check out “IBEX Mission Will View the Final Frontier of the Solar System” at the Universe Today.

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