The long-term modulation of galactic cosmic rays in the heliosphere

Abstract

The present declining phase of the solar modulation cycle is important to cosmic ray modulation because it should show to what extent gradient and curvature drifts of charged particles through the polar regions of the heliosphere determine the shape of the intensity-time (IT) profiles of long-term modulation. It is well known that standard drift models predict a clear 22-year cycle with alternating peaked-flattened 11-year profiles at all relevant energies. It has also become evident that the IT profile for the 1954 minimum modulation period was not nearly as flat as the 1976 profile. The 1965 and 1987 profiles, however, were perfectly peaked in direct correspondence to what drift models predict. The forthcoming solar minimum modulation period should therefore be most interesting: if not as flat as in 1976, it may indicate that drifts do not occur to the same extent as in 1972–1978 and that other modulation features e.g., merged interaction regions are important in determining the shape of the 22-year cycle. On the other hand, it may imply that predictions by current models are too idealistic because they are based on oversimplified assumptions e.g., a Parker magnetic field in the polar regions of the heliosphere. In this paper it will be discussed what modulation effects are important for the declining phase of the solar cycle. In this review some recent general observations of the long-term modulation of galactic cosmic rays in the heliosphere and the progress made in the modeling thereof will be summarized.