Global merged interaction regions, the heliospheric termination shock, and time-dependent cosmic ray modulation

The time-dependent cosmic ray modulation due to solar activity and global merged interaction regions is studied with a self-consistent model of the interaction of a time-varying solar wind, including pickup ions, with cosmic rays. After a study of the spatial evolution of global merged interaction regions, their different effects on local as well as global modulation of both galactic and anomalous cosmic rays are analyzed. It is found that the long-term modulation cannot be explained alone with successive step decreases in the cosmic ray intensities caused by global merged interaction regions, but that an additional long-term global variation in spatial diffusion is required. The analysis for a solar wind with pickup ions reveals that there occurs a significant upstream expansion of global merged interaction regions accompanied by a decrease of their peak density enhancement. Both effects lead to an immediate recovery of the cosmic ray intensities after a step decrease. Furthermore, the calculated response of the solar wind termination shock to the passage of global merged interaction regions shows minor changes in the structure and position so that the shock remains in a gas-dynamic (pickup ion) dominated state; that is, it is never cosmic ray dominated. This investigation also shows that the decay of global merged interaction regions is particularly fast during their interaction with the termination shock and in the region beyond, so they probably cannot be interpreted as the downstream modulation barriers discussed in the literature.