Modulation of cosmic rays in the heliosphere from solar minimum to maximum: A theoretical perspective

The modulation of galactic cosmic rays in the heliosphere seems to be domin ated by four major mechanisms: convection, diffusion, drifts (gradient, cur vature and current sheet), and adiabatic energy losses. In this regard the global structure of the solar wind, the heliospheric magnetic field (HMF), the current sheet (HCS), and that of the heliosphere itself play major role s. Individually, the four mechanisms are well understood, but in combinatio n, the complexity increases significantly especially their evolvement with time - as a function of solar activity. The Ulysses observations contribute d significantly during the past solar minimum modulation period to establis h the relative importance of these major mechanisms, leading to renewed int erest in developing more sophisticated numerical models, and in the underly ing physics, e.g., what determines the diffusion tensor. With increased sol ar activity, the relative contributions of the mentioned mechanisms change, but how they change and what causes these changes over an 11-year solar cy cle is not well understood. It can therefore be expected that present and f orthcoming observations during solar maximum activity will again produce ve ry important insights into the causes of long-term modulation. In this pape r the basic theory of solar modulation is reviewed for galactic cosmic rays . The influence of the Ulysses observations on the development of the basic theory and numerical models are discussed, especially those that have chal lenged the theory and models. Model-based predictions are shown for what mi ght be encountered during the next solar minimum. Lastly, modulation theory and modelling are discussed for periods of maximum solar activity when a g lobal reorganization of the HMF, and the HCS, occurs.