A COMPARISON OF PREDICTIONS OF A WAVY NEUTRAL SHEET DRIFT MODEL WITH COSMIC-RAY DATA OVER A WHOLE MODULATION CYCLE - 1976-1987

A comprehensive set of predictions using a newly developed, axially symmetric, steady state, drift model with a wavy neutral sheet is compared with data obtained during an entire 11 yr solar modulation cycle from 1976 to 1987. The primary emphasis of this study has been to first fit the observed 1976 spectra of protons and helium nuclei using appropriate interstellar spectra, and then to compare the observed intensity variations with those calculated using the above model and data on the neutral sheet tilt. A comparison of predictions and observations of radial intensity profiles is also made. The model accurately predicts the 1976 modulated proton and helium spectra starting with realistic interstellar spectra for these species, but only when drift effects are somewhat reduced. For the period 1981-1987, the intensity variations observed by the Mount Washington neutron monitor, the proton intensities with energy E ≥ 60 MeV, and E = 120-227 MeV observed by IMP, and those predicted by this model as a function of neutral sheet tilt, agree fairly well. These results suggest that the correlation between intensity and the observed tilt reported by other groups has a theoretical explanation in terms of a drift and wavy neutral sheet picture. The model predicts average radial gradients of particles with E ≥ 60 MeV/nucleon between 1-15 AU similar to observations before the 1981 polarity reversal. In contrast, the predicted larger gradients after the polarity reversal are not seen, in fact, the average observed gradients are smaller. However, the model can explain many of the observed features of the 11 yr cycle, and this shows that drift together with the wavy neutral sheet probably play a significant role. The large dynamic intensity decreases observed between 1978-1981 cannot be accommodated in this model because of the assumption of a steady state and will have to be addressed in a more comprehensive time-dependent model.