Solar Energetic Particle Variations

The population of energetic particles in the heliosphere changes from solar maximum to solar minimum. The ultimate driver for those variations is the Sun. Solar variability is reflected in the dynamics of the large-scale structure of the heliosphere, the solar output of energetic particles and most definitively in the intensity, energy and composition of the population of energetic particles observed by spacecraft and ground-based detectors. We describe the classification, origin, intensity and properties of the solar energetic particle events, in terms of both energetic ions and electrons. We review the long-term databases created to compare the occurrence frequency and intensity distribution of events among the solar cycles. Models for the production of the largest gradual solar energetic particle events are discussed.