Abstract
The long term modulation of galactic cosmic rays in the heliosphere deals with the causes of the 1 l-year and 22year solar cycles, while short term modulation is associated with effects of the order of a few solar rotations. During the past decade, with four spacecraft (Pioneer 10 & 11, Voyager 1 & 2) at various relatively large distances from the Sun, and with Ulysses inside of 5 AU and IMP 8 at Earth, the emphasis has changed to understand modulation as being applicable to three distinctive areas of the heliosphere. They are conveniently called the inner, middle and outer heliosphere. The outer heliosphere is about studying the effects of the configuration of the heliosphere, its interfaces with the interstellar medium and the termination shock - all aspects of great importance to cosmic ray modulation. The middle heliosphere is the region where shorter term effects like corotating regions and the effects of coronal mass ejections lose their characteristic features and merge into bigger entities of different scales which have profound effects on medium to long term modulation. The inner heliosphere that Ulysses explores is characterized by many short term events, superimposed on the long-term baseline, which have apart from their strong temporal and radial dependence easily recognizable latitudinal and azimuthal features. Compatibility studies between the results of modelling and observations at Earth and beyond have been done with reasonable success over the years. This review focuses on theory and modelling, and the understanding of the basic mechanisms of galactic cosmic ray modulation in the heliosphere, with emphasis on long-term modulation cycles.
Published Version
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