During September 1979, Helios B (HB) and Voyager 1 (V1) were near 0.85 AU and 6.2 AU, respectively, and they were radially aligned with respect to the sun within 20¿. A slow transient stream, two corotating streams, and two unusually fast transient streams passed HB during a 17-day interval in that order. (A stream is defined here as a flow in which the speed is always greater than 475 km/s.) As the streams moved outward from HB to V1, they coalesced to form a single large ''compound stream'' which moved past V1 in 20 days. The compound stream was produced by motion of fast plasma away from slower plasma behind and by the interaction of the fast streams with slower streams ahead. The maximum speed observed was 1270 km/s at HB and 860 km/s at V1.

Closely associated with this compound stream was one of the largest solar energetic particle events (protons 0.4--69 MeV) observed beyond 5 AU. The maximum intensity was in the region of increasing speed. The intensity-time profile of the solar energetic particles was closely related to the speed profile. There is evidence of channeling and trapping of particles in a region following the highest speeds, where the magnetic field was strong and the magnetic field lines were nearly radial. The high-energy protons (>150 MeV) and MeV electrons are believed to be accelerated in solar flare events. Interplanetary acceleration appears to play a major role in the low-energy particle population. The galactic cosmic ray intensity remained low throughout the passage of the compound stream, recovering slowly as the speed decreased. The long ''recovery time'' is thus related to the large extent of the flow.