The Protecting Cocoon of the Solar System: NASA Mission discovers unexpected structures

The solar wind engulves our solar system like a cocoon: This continuous plasma flow that emanates from the Sun is protecting us from the interstellar medium, especially from the cosmic rays. The boundary of this cocoon is a long-standing topic of space research.

Now the NASA spacecraft IBEX (Interstellar Boundary Explorer) has, for the first time, detected energetic hydrogen atoms from this region resulting in a surprise: The measurements indicate entirely unexpected structures in the flux of these particles.

“All scientists have, so far, modelled this outer boundary without an interstellar magnetic field – nobody has expected its strong influence”, states PD Dr. Horst Fichtner (Institut für Theoretische Physik IV der RUB). He and his international colleagues present these observations and new improved models of this cocoon in two publications in Science.

Electron exchange at the boundary of the heliosphere

The IBEX spacecraft, launched in October last year into Earth orbit, has pointed its novel detectors away from Earth into outer space and records how many energetic hydrogen atoms arrive per time interval from a given direction. Step by step it has scanned the whole sky and provided the first all-sky map of this particle flux, which allows to infer the physical conditions at the outer boundary of our solar system: At the outer edge of the heliosphere – the plasma cocoon – the solar wind plasma interacts with the interstellar medium. The solar wind consists partly of fast protons, the interstellar medium to a large fraction of slow hydrogen atoms. There is a specific probability that close encounters result in a transfer of an electron from the slow hydrogen atom to the fast proton. “Thereby, the two particles exchange their roles”, explains Dr. Fichtner, “the fast proton transforms into a fast hydrogen atom and vice versa. We can measure the result of this transformation.”

Modelling without the interstellar magnetic field

The all-sky map measured by IBEX surprised the scientists, however. It showed only partly the theoretically predicted structures, which were obtained under the assumption that the flux of energetic atoms is mainly determined by the solar wind and not by the interstellar magnetic field. Instead an intensity ribbon – resulting from relatively many exchange processes – showed up that stretches “diagonally” across the all-sky map. “Meanwhile, we know why”, explains Horst Fichtner. “This ribbon fits to the interstellar magnetic field. The latter has been neglected in the models so far.”

New scenarios

In the second Science publication the researchers now develop scenarios to explain the newly observed data. “We assume that the magnetic field plays a dynamic role leading to a compression of the heliosphere at its boundary”, says Fichtner. The magnetic field forces the plasma flow from the Sun to decelerate resulting in an accumulation of particles. Thereby, the probability for “collisions” and, hence, that for electron transfer increase.

Waiting for further measurements

“These first results of the IBEX mission are a milestone on the way to a deeper understanding of the heliosphere and its galactic environment, which also determines the conditions for life on Earth.”, comments Horst Fichtner, who heads a heliospheric research group. The detailed insights into the physics of the heliosphere can be transferred to other stars and help to understand the significance of astrospheres for extrasolar planets. The IBEX observations also show first indications for a time variability of the flux of neutral atoms and, thus, of the structure of the heliosphere: “Based on our models this is what we expect as a consequence of solar activity cycle”, says Dr. Fichtner. The confirmation of such variation can, however, only result from measurements over a longer period. “The further measurements by IBEX, which will be operating for at least two years but, probably, far more, are eagerly expected!”

IBEX

IBEX is the latest in NASA's series of low-cost, rapidly developed Small Explorers space missions. Southwest Research Institute in San Antonio, TX, leads and developed the mission with a team of national and international partners. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the Explorers Program for NASA's Science Mission Directorate in Washington.

Titles

N. A. Schwadron, M. Bzowski, G. B. Crew, M. Gruntman, H. Fahr, H. Fichtner, P. C. Frisch, H. O. Funsten, S. Fuselier, J. Heerikhuisen, V. Izmodenov, H. Kucharek, M. Lee, G. Livadiotis, D. J. McComas, E. Moebius, T. Moore, J. Mukherjee, N. V. Pogorelov, C. Prested, D. Reisenfeld, E. Roelof, and G. P. Zank: Comparison of Interstellar Boundary Explorer Observations with 3-D Global Heliospheric Models. In: Science Express, Published online October 15 2009; 10.1126/science.1180986 (Science Express Reports)

D. J. McComas, F. Allegrini, P. Bochsler, M. Bzowski, E. R. Christian, G. B. Crew, R. DeMajistre, H. Fahr, H. Fichtner, P. C. Frisch, H. O. Funsten, S. A. Fuselier, G. Gloeckler, M. Gruntman, J. Heerikhuisen, V. Izmodenov, P. Janzen, P. Knappenberger, S. Krimigis, H. Kucharek, M. Lee, G. Livadiotis, S. Livi, R. J. MacDowall, D. Mitchell, E. Mobius, T. Moore, N. V. Pogorelov, D. Reisenfeld, E. Roelof, L. Saul, N. A. Schwadron, P. W. Valek, R. Vanderspek, P. Wurz, and G. P. Zank: Global Observations of the Interstellar Interaction from the Interstellar Boundary Explorer (IBEX), In: Science Express, Published online October 15 2009; DOI: 10.1126/science.1180906 (Science Express Reports)

Further Information

PD Dr. Horst Fichtner, Institut für Theoretische Physik der Ruhr-Universität, 44780 Bochum, Tel. 0234/32-23786, E-Mail: hf@tp4.rub.de