Voyager Observations of Anomalous Cosmic Rays in the Heliosheath

The two Voyager spacecraft are now exploring the heliosheath. Voyager 1 (V1) crossed the termination shock in December 2004 and Voyager 2 (V2) crossed in August 2007. Recently, the intensities of anomalous cosmic rays (ACRs) have exhibited intriguing changes. At V1, the ACR He and O intensities have been declining since ~mid-2010. The decline suggests that the ACR source is not radially beyond 114 AU and that ACRs may be leaking out through the heliopause. If so, the V1 spacecraft may be close to entering interstellar space for the first time. At V2, the ACR He and O intensities began to rapidly increase near the beginning of 2010 and at several energies now exceed the intensities at V1. The V2 intensities are also higher than the peak intensities at V1 during 2010. The excess is nearly 100% for ACR O with energies 1.9-8.5 MeV/nuc and ~25% for ACR He with energies 6.2-12.8 MeV/nuc. These observations suggest that the ACR source intensity in the southern part of the heliosphere is greater than that in the north, or that the source of ACRs is along the flank or tail of the heliosphere. The latest observations will be presented at the meeting. This work was supported by NASA under contract NAS7-03001 and grant NNX10AE45G.