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IBEX maps edge of
Solar System

Posted: October 15, 2009

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The first all sky maps from NASA's Interstellar Boundary Explorer (IBEX) reveal intense, theory-defying interactions occurring between the edge of the Solar System and interstellar space.

“The IBEX results are truly remarkable, with emissions not resembling any of the current theories or models of this never-before-seen region,” says IBEX principal investigator David McComas. “We expected to see small, gradual spatial variations at the interstellar boundary, some ten billion miles away. However, IBEX is showing us a very narrow ribbon that is two to three times brighter than anything else in the sky.”

This image illustrates one possible explanation for the bright ribbon of emission seen in the IBEX map. The galactic magnetic field shapes the heliosphere as it drapes over it. The ribbon appears to trace the area where the magnetic field is most parallel to the surface of the heliosphere (the heliopause). Image: Southwest Research Institute.

The Sun blows out a continuos stream of charged particles – the solar wind – that inflates a giant bubble around the Solar System known as the heliosphere. At the same time, neutral particles are drifting in from interstellar space, and as the solar wind races outwards, it sweeps up newly formed “pick-up ions,” which arise from the ionization of these neutral particles. Energetic neutral atoms (ENAs) are produced in the boundary between the heliosphere and the local interstellar medium, and IBEX has measured these traveling at speeds of roughly half a million to two and a half million miles per hour.

For around a year, IBEX has been monitoring the global interaction of particles at the interstellar boundary. ENAs propagate in from all parts of the boundary and, by detecting their directions of arrival, IBEX can build up maps of this invisible region over a broad range of energies.

The Voyager spacecraft provided the first point measurements of interactions at the interstellar boundary at two different locations, but IBEX demonstrates the importance of global observations. In tonight’s press conference announcing the results, the scientists liken the Voyager spacecraft to two local weather stations, but the IBEX spacecraft to a global weather satellite, which fills in the gaps.

“The most astounding feature in the IBEX sky maps – the bright narrow ribbon – snakes through the sky between the Voyager spacecraft, where it remained completely undetected until now,” says McComas. Neither Voyager spacecraft detected this high energy band, showing just how confined it is.

Accurate timing of the incoming ENAs allows the IBEX team to obtain a higher resolution in the latitudinal direction. The inset at right shows some of the fine detail of the ribbon. Image: Southwest Research Institute.

Zooming in on segments of the ribbon shows fine structures which suggests that ion densities may be significantly enhanced in highly localized regions at the interstellar boundary.

“Our expectation was to see small variations – tens of percents – but instead we’re seeing 200-300 percent variations in intensities,” says McComas. “It’s a shocking result and not entirely understood. It shows what we thought before is not right and that we need to start over.”

It is still early days in terms of interpreting the data, but because the ribbon appears to be ordered by the direction of the local interstellar magnetic field outside the heliosphere, the new observations suggest that the interstellar environment has far more influence on the structure of the heliosphere than anyone previously believed. “The external magnetic field is oriented in such a way that it correlates with the ribbon; it’s pushing in from the south and at the sides and where it’s most bent is where the ribbon is lying. Somehow the external magnetic field is playing a role in this interaction, but we don’t know why it produces the 200-300 percent higher fluxes than surrounding regions.”

Theoreticians and modellers are now trying to tie all the pieces together to build up a comprehensive idea of what is going on, but the journey may only just be beginning. “IBEX is part way through its second map, and we see the ribbon but it might have evolved since the last sky map so it will be fascinating to watch it change,” concludes McComas.

Understanding the interstellar space environment is crucial for progressing manned exploration of the Solar System, since the interstellar boundary shields the Solar System from the majority of galactic cosmic ray radiation.