COSMIC-RAY MODULATION - VOYAGER-2 OBSERVATIONS, 1987-1988

We analyze the intensity profile of cosmic rays > 70 MeV observed by Voyager 2 and its relation to the interplanetary magnetic field and plasma at the beginning of the new modulation cycle from day 190, 1987 to day 345, 1988 in the region from 23.3 AU to 27.8 AU. The cosmic ray intensity profile was approximately a series of four plateaus separated by three steps in which the intensity dropped abruptly. Each step was associated with a region in which the magnetic field, density, and temperature were higher than average. The plateaus were associated with regions in which the magnetic field was alternately strong and weak. To first approximation, one can picture the solar wind within 200 AU during this interval as consisting of three shells between which the flow was quasi-periodic with a approximately 26 day periodicity. This picture is an oversimplification, since the large-scale magnetic field was approximately a multifractal. The latitudinal extent of the shells in the northern hemisphere was probably < 33-degrees, since no steps were observed by Voyager 1. Drift motions might play a role (1) during the recovery phase, just prior to the onset of the new modulation cycle, (2) in the plateau regions between the shells, (3) within the shells where drifts in various directions might mimic diffusion, and (4) close to 1 AU, where large regions of intense magnetic fields have not yet formed. However, the principal decreases in the cosmic ray intensity in the outer heliosphere during 1987 and 1988 were associated with the passage of broad regions of intense magnetic fields, consistent with the diffusion/convection model.