Source and consequences of a large shock near 79 AU

In March 2006, Voyager 2 (V2) observed a large interplanetary (IP) shock near 79 AU followed by a merged interaction region (MIR). This shock is comparable to the shock observed by V2 at 65 AU in October 2001; these two shocks are the largest observed by V2 since 1991 when V2 was at 35 AU. This shock provides the first opportunity to compare the plasma structure in an IP shock and MIR with the energetic particle fluxes in the termination shock (TS) foreshock region. The flux of >0.5 MeV particles observed by V2 decreased after the shock; the shock and MIR probably pushed the TS outward so that the foreshock region moved outside the distance of V2. The >70 MeV cosmic ray ions decreased in the MIR, probably due to the reduced inward transport caused by the enhanced magnetic field. We model two possible sources of this shock, fast streams from polar coronal holes and coronal mass ejections (CMEs); these sources, when combined, provide a reasonable match to the V2 data.