Studying the heliosphere and its interactions with the interstellar medium isn’t easy, which is one among many reasons we follow the fortunes of the Voyager probes with such continuing fascination. They’re pushing up against the boundary between the Sun’s local ‘bubble’ and deep space beyond, where charged particles from the solar wind are no longer a factor and the deeper rhythms of the galaxy take hold. Now our other probe of this exotic region is back in the news in a new paper. IBEX (the Interstellar Boundary Explorer) is telling us much about how our system interacts with the interstellar medium and the effects of the galactic magnetic field upon the heliosphere.
IBEX has provoked much discussion in these pages — I was amazed to see I had written fully thirteen articles on the mission in the last six years, going back to pre-launch speculations. The mission caught my eye because it was the first ever sent with the express purpose of studying the outer edges of the Solar System. The Voyagers, obviously, are powerful tools, but exploring the heliopause was not their primary mission, nor are they optimized for this kind of work. What IBEX gives us will tell us much about the strength of the interactions in the heliosheath at system’s edge.
Recall that despite its target, the IBEX spacecraft itself orbits the Earth (with an apogee of 322,000 kilometers and a perigee of 16,000 kilometers), studying the heliosphere’s boundary by examining energetic neutral particles (ENAs) that help to define it. These ENAs are swept up and scattered by the solar wind in the boundary between the edge of the heliosphere and interstellar space. IBEX has created maps based upon those ENAs that go on to pass near the Earth, telling us much about the structure of the heliosphere, and discovering that the ENA emissions do not resemble what previous models had predicted.
IBEX principal investigator David J. McComas (SwRI) commented on the most newsworthy of the IBEX results back in 2009:
“The IBEX results are truly remarkable, with emissions not resembling any of the current theories or models of this never-before-seen region. 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.”
Image: This graphic 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). Credit: SwRI.
The ribbon feature was an unanticipated finding, one that required the development of a method to separate the ribbon from background emissions to obtain clearer resolution. In the new paper, Nathan Schwadron (University of New Hampshire) describes the process. Schwadron says that isolating and separating this ribbon of energy from the IBEX data was ‘like pulling the drapes from our window to discover the landscape at the edge of the solar system.’
One useful result of this work is that by learning more about the physical properties of the heliosphere, we will have a better idea how galactic cosmic rays operate both outside and inside the heliosphere, with obvious implications for planetary radiation environments not only in our system but in those around other stars. This is a murky area, and while we know that the most powerful galactic cosmic rays can penetrate the Earth’s magnetic fields and atmosphere, we have much to learn about how the heliosphere protects the Solar System from less powerful background cosmic rays.
The maps IBEX is making are probing unexplored terrain and, as the energy ribbon shows, contain surprising features.
“There are many theories about how the ribbon is created, and we don’t understand exactly what we’re seeing but it seems to be telling us something about how the local galactic magnetic field interacts with the heliosphere. This galactic magnetic field may be a missing key to understanding how the heliosphere protects the solar system from galactic cosmic rays.”
Other features of interest from the IBEX data are a ‘tail’ of emissions at the boundary — one that seems to be deflected in the direction of the galactic magnetic field — and the ‘nose’ of the heliosphere, which Schwadron likens to the “bow wave in front of a ship, which shows us how our motion through the galaxy compresses and deflects the material of the local galactic medium around our heliosphere.” But it’s worth noting that what IBEX has shown us so far differs greatly from what mission planners had expected. The crucial role of the galactic magnetic field now becomes apparent as our planetary system interacts with the galaxy.
The paper is Schwadron et al., “Separation of the Interstellar Boundary Explorer Ribbon from Globally Distributed Energetic Neutral Atom Flux,” Astrophysical Journal 731 (10 April 2011), p. 56 (abstract).