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Enhancements of energetic particles near the heliospheric termination shock


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 heliosphere1,2. 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.

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Figure 1: Time histories (5-day moving averages) of Voyager 1 cosmic ray intensities.
Figure 2: Time histories (5-day moving averages) of Voyager 1 (V1) and Voyager 2 (V2) cosmic-ray intensities.
Figure 3: Voyager 1 and Voyager 2 energy spectra (2002/209–2002/364).


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We thank J. D. Richardson and colleagues for making the Voyager 2 solar wind data available at 〈〉. We also thank J. R. Jokipii, F. C. Jones, H. Moraal, V. Ptuskin and R. Caballero-Lopez for discussions on shock acceleration processes and particle flow in the vicinity of the acceleration region.

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Correspondence to Frank B. McDonald.

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McDonald, F., Stone, E., Cummings, A. et al. Enhancements of energetic particles near the heliospheric termination shock. Nature 426, 48–51 (2003).

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