The purpose of this chapter is to lay out the basic physical foundations of cosmicray transport in space. We will discuss the basic physics of the cosmic-ray transport equation, first written down by Parker (1965). This equation is remarkably robust and is widely used to study cosmic-ray transport in the solar system and the interstellar medium. The chapter starts with a general background on cosmic rays in the solar system. The transport equation itself is not formally derived, but the basic physical processes that lead to the various terms in this equation are discussed in detail. We will also address what assumptions are made about this equation and under what conditions it is applicable. At the end of this chapter, we provide a few applications related to specific heliophysics phenomena, such as the propagation of solar-energetic particles in the inner heliosphere, the modulation of galactic cosmic rays, and the drift motions of cosmic rays in the solar system.

Cosmic rays in the solar system

A fundamental and outstanding problem in astrophysics concerns the origin of high-energy charged particles in space. This problem has been known since the early 1900s when Victor Hess performed his famous electroscope experiments on balloon flights to demonstrate that the excess atmospheric radiation increased with increasing distance from the Earth's surface. This led to the discovery of cosmic rays. In the 1920s Chapman and Ferraro considered the effect on Earth of localized and intermittent streams of corpuscular radiation. It was later determined that the solar corpuscular radiation had two components: one that was steady, now known as the solar wind, and the other intermittent, that we now know to be solar energetic particles.