• cosmic rays;
  • heliosheath;
  • protons

The Voyager 1 spacecraft is exploring the outer heliosheath past about 111 astronomical units from the Sun. The heliosheath is the region where the outgoing solar wind is slowed by the interstellar medium. The Voyager spacecraft have been sending back interesting new information about the structure of this previously uncharted boundary region at the edge of the solar system. Webber et al. now report that Voyager 1 recently observed two sudden increases in the intensity of low-energy cosmic ray electrons as it traveled farther from the Sun. At the outer boundary of the heliosheath, the electron intensity is usually assumed to be equal to that in interstellar space, outside the heliosphere. The authors suggest that the sudden changes in electron intensity are evidence of significantly different regions in the structure of the outer heliosheath and that as of early 2012, Voyager 1 had not quite reached the undisturbed interstellar medium outside of the heliosheath. (Geophysical Research Letters, doi:10.1029/2012GL051171, 2012)