THE ROLE OF MERGED INTERACTION REGIONS AND DRIFTS IN THE HELIOSPHERIC MODULATION OF COSMIC-RAYS BEYOND 20-AU - A COMPUTER-SIMULATION

The time-dependent, axially symmetric transport equation of cosmic rays in the heliosphere was solved numerically using the Voyager 2 magnetic field measurements to simulate merged interaction regions (MIRs) and rarefaction regions (RRs) for the period 1985-1989. Concurrently, the wavy neutral sheet was used as a time-dependent, drift parameter. The two approaches were first modeled separately and then in combination in this exploratory study. The drift approach proved to be the more successful for 1985 to early 1988 because it could reproduce the intensity levels, the factor modulation, and latitudinal gradients for 1 GeV protons at 23 AU without difficulties. The MIR approach used here, on the other hand, could not reproduce, in a straightforward manner, any of these observations unless the assumed modulation boundary was taken at 150-200 AU from the Sun, or when the local interstellar spectrum was made considerably higher than what is presently considered reasonable. With this approach, it was found that the latitudinal span (or, more generally, the spatial extent) of the MIRs became an important modulating parameter in addition to those already well established. Although this very basic approach was less successful for this particular period, it did exhibit important features that the drift approach lacked. Indications were found that it would be more successful for other periods, and that a more advanced approach would do better overall. However, the combination of the two approaches shows exciting prospects in explaining the complete 11 yr and 22 yr modulation cycles. It is concluded that for the period 1985 through 1987 drifts probably were responsible for the ''baseline'' of modulation. MIRs became progressively more important after 1987.6 and may be dominant when solar activity as measured by the ''tilt angle'' of the wavy neutral sheet exceeds moderate levels (35-degrees +/- 5-degrees), as happened before 1985 and after mid-1988.