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Science 23 September 2005:
Vol. 309. no. 5743, pp. 2017 - 2020
DOI: 10.1126/science.1117684
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Reports

Voyager 1 Explores the Termination Shock Region and the Heliosheath Beyond

E. C. Stone,1* A. C. Cummings,1 F. B. McDonald,2 B. C. Heikkila,3 N. Lal,3 W. R. Webber4

Voyager 1 crossed the termination shock of the supersonic flow of the solar wind on 16 December 2004 at a distance of 94.01 astronomical units from the Sun, becoming the first spacecraft to begin exploring the heliosheath, the outermost layer of the heliosphere. The shock is a steady source of low-energy protons with an energy spectrum ~E–1.41 ± 0.15 from 0.5 to ~3.5 megaelectron volts, consistent with a weak termination shock having a solar wind velocity jump ratio . However, in contradiction to many predictions, the intensity of anomalous cosmic ray (ACR) helium did not peak at the shock, indicating that the ACR source is not in the shock region local to Voyager 1. The intensities of ~10–megaelectron volt electrons, ACRs, and galactic cosmic rays have steadily increased since late 2004 as the effects of solar modulation have decreased.

1 California Institute of Technology, Pasadena, CA 91125, USA.
2 Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742, USA.
3 NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA.
4 Department of Physics and Astronomy, New Mexico State University, Las Cruces, NM 88003, USA.

* To whom correspondence should be addressed. E-mail: ecs{at}srl.caltech.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)