In September 1979 an unusual large-scale interplanetary compound stream was observed in data from the plasma and magnetic fields experiments of Voyager 1 at a heliocentric distance of 6.2 AU. Two interaction regions were observed by Voyager 1, each lasting more than 4 days, in which the total pressure was nearly 100 times the ambient pressure. Speeds of greater than 450 km/s were observed for more than 27 days. A strong forward shock FD with a shock speed of 960 km/s and a density ratio of 3.85 was present at the front of the second interaction region, and two reverse shocks, R1 and R2, were at the end of the first interaction region. A major restructuring of the flow took place over the distance of 3.3 AU between Voyager 1 and Pioneer 11, which was nearly radially aligned with Voyager 1.

During the restructuring the two interaction regions observed at 6.2 AU coalesced to form a new single structure (a ''merged interaction region'') at 9.5 AU. This paper presents a simulation study of the evolution and interaction of solar wind structures to explain the observed restructuring process. The computer simulation is based on an unsteady, one-dimensional, one-fluid, MHD model and uses the plasma and magnetic field data from Voyager 1 at 6.2 AU as the input function. The model shows that the forward shock FD passed through the reverse shocks R1 and R2 and into the first interaction region, becoming weaker in each of these interactions. The reverse shocks coalesced to form a stronger reverse shock R; thus the shock signature changed from R1-R2-FD to FD-R between 6.2 and 9.5 AU. The major stream structures at 9.5 AU predicted by the simulation model agree well with those directly observed rom Pioneer 11.