Energetic particles in our heliosphere are key in the understanding of the evolution and the current status of our solar system. The energy distributions of ions and electrons are widely used to understand acceleration phenomena in the heliosphere and in the vicinity of planets and moons. Knowing the energy spectra of those particles can tremendously help in the characterization of plasma sources and sinks of many different environments in the Milky Way and beyond. The knowledge of those energetic particle distributions in fact is essential in many fundamental plasma physics problems. They are a very nice tool to investigate acceleration mechanisms, geochemistry and solar-system evolution, atmospheric composition and solar-system evolution, and last but not least energetic particles are important in studying the configuration and dynamics of planetary magnetospheres and the interaction of magnetospheric plasma with their moons, rings, neutral gas, or dust clouds.

Energetic particles in the energy range between 1 eV to 1020 eV can be found everywhere in our solar system as sketched in Fig. 12.1. Their sources can be either outside our solar system from galactic or extra-galactic interstellar space or inside our solar system from the Sun or the planets or created in various acceleration processes in interplanetary space or inside planetary magnetospheres, i.e., at interplanetary shocks, corotating interaction regions, planetary bow shocks or at the termination shock of the heliosphere. Types of energetic particles range from electrons to charged atoms and molecules to neutral atoms and molecules as well as dust particles. Figure 12.2 shows the particle intensity versus energy spectra of various types of energetic particles (left) and for cosmic rays (right).

This chapter focuses on the description of charged energetic particle populations from the Sun and particle distributions inside planetary magnetospheres (including galactic cosmic rays (GCRs) as a potential source of charged particles in planetary radiation belts). Energetic particles from acceleration processes at shocks are reviewed in Ch. 7 of Vol. II.

In order to set the scene for later sections and without going into much detail we introduce the motion of charged particles in a magnetic field. For further details and equation derivations the reader is referred to books like Roederer (1970) or Walt (1994). A excellent text used for this chapter is Kallenbach et al. (2006).