Voyager 1 in the Vicinity of the Solar Wind Termination Shock

The termination shock marks the abrupt slowing of the solar wind from supersonic to subsonic speeds as it approaches the edge of the heliosphere. The shock likely accelerates anomalous cosmic rays and reaccelerates galactic cosmic rays and other solar and interplanetary ions. In mid 2002 when Voyager 1 was at 85 AU, it observed an unusual enhancement of MeV ions and electrons that was not present at Voyager 2 at 68 AU. Although some aspects of the energetic particle data suggest that Voyager 1 entered the subsonic flow of the heliosheath at that time, the anomalous cosmic ray intensity remained lower than expected at the shock and the interplanetary magnetic field did not show the compression expected as the wind slows. These and other aspects such as strong field aligned beaming suggest an unexpected complexity in the vicinity of a termination shock that is more dynamic than anticipated. Our understanding of this new region of the heliosphere should continue to increase as Voyager 1 surfs along the shock over the next several years.