Frank McDonald
University of Maryland
Where were the Anomalous Cosmic RaysWhen Voyager 1 Crossed the Termination Shock
On December 16, 2004 at 94 AU (34°N) Voyager 1 crossed the heliospheric termination shock (TS) and entered the region of the heliosheath. At the TS the observed intensities of galactic and anomalous cosmic rays (GCRs and ACRs) were much less than had been predicted and from what had been observed over the 1996-98 solar minimum period and for the termination shock particle events (TSP) that were first detected after 2002.5. It was expected that the ACR source spectra would be present at the TS along with a small GCR increase from reacceleration at the TS. Now after almost two years of V1 observations in the heliosheath and of V2 TSP increases, we are beginning to understand the low ACR and GCR intensities at the TS on 16 Dec. 2004. There appear to be three major factors: (a) The passage of the large interplanetary transients produced by the Oct./Nov. 2003 Halloween events and subsequent activity over the following several months; (b) The V1/2 data from solar maximum to the present suggest a long term solar cycle variation of the ACRs at the TS. The nature of the process that would produce such a change is not understood at this time; (c) There is a 22 year variation in the lower energy ACRs (ie 4-20 MeV/n He+) associated with the polarity reversal of the interplanetary B field at the TS over solar maximum. Implicit in this discussion is the assumption that the TSP events provide a reasonable estimate of the ACRs and GCRs at the TS close to the V1/2 heliolatitudes.