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

Correspondence to: Frank B. McDonald¹ Email: fm27@umail.umd.edu

Abstract

The spacecraft Voyager 1 is at a distance greater than 85 au from the Sun, in the vicinity of the termination shock that marks the abrupt slowing of the supersonic solar wind and the beginning of the extended and unexplored distant heliosphere^{1, 2}. This shock is expected to accelerate 'anomalous cosmic rays'³, as well as to re-accelerate Galactic cosmic rays⁵ and low-energy particles from the inner Solar System⁴. Here we report a significant increase in the numbers of energetic ions and electrons that persisted for seven months beginning in mid-2002. This increase differs from any previously observed in that there was a simultaneous increase in Galactic cosmic ray ions and electrons, anomalous cosmic rays and low-energy ions. The low-intensity level and spectral energy distribution of the anomalous cosmic rays, however, indicates that Voyager 1 still has not reached the termination shock. Rather, the observed increase is an expected precursor event. We argue that the radial anisotropy of the cosmic rays is expected to be small in the foreshock region, as is observed.

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

Correspondence to: Frank B. McDonald¹ Email: fm27@umail.umd.edu