The simulated features of heliospheric cosmic-ray modulation with a time-dependent drift model. I. General effects of the changing neutral sheet over the period 1985-1990

The heliospheric modulation of galactic cosmic rays is simulated with a time-dependent drift model with emphasis on the effects of the wavy heliospheric neutral sheet (HNS) as a function of time during the period 1985-1990. This was done by assuming the outward propagating HNS as the only time-dependent parameter which made it possible to establish to what extent the HNS could have been dominating the modulation process during this period. The model predicted a clearly defined time difference between the time when minimum modulation occurred at Earth and at radial distances farther away from the Sun, indicating that the HNS plays an important part in establishing this observed time delay. The movements of the Voyager 1 (V1) and 2 (V2) and Pioneer 10 (P10) spacecraft were simulated in order to calculate instantaneous radial and latitudinal gradients. The time dependencies of these gradients were found to follow the observations well; the radial gradient between the simulated P10 and 1 AU varied from 2.7% AU-1 in the early simulated 1985 to 2.1% AU-1 at minimum modulation after when it increased rapidly to 3.1% AU-1 reaching a maximum of almost 4% AU-1 at maximum HNS waviness. The calculated radial gradient between the simulated V2 and P10 was found smaller, varying from ∼2.5% AU-1 in 1985, to 1.1% AU-1 in 1987, to ∼3.2% AU-1 in 1989, indicating that the predicted radial gradient decreased with increasing radial distance during the period considered. Unfortunately, the model predicted the modulation and therefore the radial gradients too large during periods of high solar activity. The predicted instantaneous "latitudinal gradient" between the simulated movements for V1 and V2 was found to vary between a small positive value in early 1985 to -0.85% deg-1 at minimum modulation, with a rapid increase after this period to +0.8% deg-1 in 1989. This kind of compatibility with observations supports therefore the view that the HNS was probably responsible for establishing the underlying latitude-dependence of heliospheric modulation for the period and energies considered. The role of K⊥ and a modification of the heliospheric polar magnetic field were also studied and showed that a small K⊥ ≈ 6.6 × 1019 cm2 s-1 for 1 GV particles at Earth was required to obtain reasonable latitudinal gradients. The modification to the Parker spiral field was found to play a moderate role but might become more important with increased solar activity. Since the model predicted the modulation after 1987 too large due to the extraordinary rapid increase in the HNS waviness, we came to the conclusion that additional time-dependent parameters or features would have to be incorporated to explain this period. The model study nevertheless contributes to the growing evidence that the HNS cannot be ignored, especially during periods of low to moderate solar activity, when the modulation of cosmic rays is described.