A time-dependent drift model for the long-term modulation of cosmic rays with special reference to asymmetries with respect to the solar minimum of 1987

Abstract

We have developed a time-dependent drift model with a simulated wavy neutral sheet in which changes in the waviness of the sheet propagate radially outward at the solar wind velocity. Using an asymmetric rate of change in the tilt angle, α, of the wavy neutral sheet (apparent in observations with respect to the solar minimum of 1987) in the model, we can explain the observed lesser response of the cosmic-ray intensity to temporal changes in α after 1987 as well as the somewhat asymmetric behavior of the intensity-time profile with respect to 1987 which seem beyond the scope of steady state drift models. <P />In order to put these novel time-dependent drift effects to the test, we incorporate actual neutral sheet tilt angles in the model and compare the calculated intensities with the intensities (E ≥ 60 MeV) observed on the IMP 8 spacecraft from 1976 to 1989. Promising correlation is achieved, especially during periods of lower solar activity, giving credibility to a time-dependent approach to drift models as being necessary to explain the long-term modulation of cosmic rays.