Letters to Nature

Nature 426, 45-48 (6 November 2003) | doi:10.1038/nature02068; Received 27 May 2003; Accepted 18 September 2003

Voyager 1 exited the solar wind at a distance of approx85 au from the Sun

S. M. Krimigis1, R. B. Decker1, M. E. Hill2, T. P. Armstrong3, G. Gloeckler2, D. C. Hamilton2, L. J. Lanzerotti4,5 & E. C. Roelof1

  1. Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland 20723, USA
  2. Department of Physics, University of Maryland, College Park, Maryland 20742, USA
  3. Fundamental Technologies, 2411 Ponderosa, Suite A, Lawrence, Kansas 66046, USA
  4. Bell Laboratories, 600 Mountain Avenue, Building 1E-439, Murray Hill, New Jersey 07974, USA
  5. Center for Solar Terrestrial Research, New Jersey Institute of Technology, Newark, New Jersey 07102, USA

Correspondence to: S. M. Krimigis1 Email: tom.krimigis@jhuapl.edu

The outer limit of the Solar System is often considered to be at the distance from the Sun where the solar wind changes from supersonic to subsonic flow1. Theory predicts that a termination shock marks this boundary, with locations ranging2 from a few to over 100 au (1 au approximately 1.5 times 108 km, the distance from Earth to the Sun). 'Pick-up ions' that originate3, 4 as interstellar neutral atoms should be accelerated to tens of MeV at the termination shock, generating anomalous cosmic rays5, 6, 7. Here we report a large increase in the intensity of energetic particles in the outer heliosphere, as measured by an instrument on the Voyager 1 spacecraft. We argue that the spacecraft exited the supersonic solar wind and passed into the subsonic region (possibly beyond the termination shock) on about 1 August 2002 at a distance of approx85 au (heliolatitude approx34° N), then re-entered the supersonic solar wind about 200 days later at approx87 au from the Sun. We show that the composition of the ions accelerated at the putative termination shock is that of anomalous cosmic rays and of interstellar pick-up ions.