1 Introduction

Galactic cosmic rays encounter a turbulent solar wind with an embedded heliospheric magnetic field (HMF) when entering the heliosphere. This leads to significant global and temporal variations in their intensity and in their energy as a function of position inside the heliosphere. This process is identified as the solar modulation of cosmic rays (CRs). For this review, CRs are considered to have energies above 1 MeV/nuc and come mainly from outside the heliosphere, with the exception of the anomalous component of cosmic rays (ACRs) which originates inside the heliosphere.

The purpose of this overview is to explain and discuss progress of this process with its intriguing facets. The modulation of CRs is considered to happen from below ∼ 30 GeV/nuc. The review also includes some aspects of ACRs but solar energetic particles, compositional abundances and isotopes are not included. The fascinating origin of CRs and acceleration in galactic space and beyond are not discussed. The emphasis is on the global features of CR modulation and on the causes of the observed 11-year and 22-year cycles and phenomena such as charge-sign dependent modulation. Shorter-term CR variations, on scales shorter than one solar rotation, are not part of this review. Space weather and related issues are reviewed by Shea and Smart (2012), amongst others.

The spotlight is first on the global features of the heliosphere and how it responds to solar activity. It is after all this extensive volume in which solar modulation takes place, mainly determined by what happens on and with the Sun. Transport and modulation theory is explained and the recurrent behaviour that CRs exhibit in the heliosphere is discussed within this context. Major predictions and accomplishments based on numerical modeling and some observational highlights are given. This overview is meant to be informative and didactic in nature.

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