Particle diffusion through random increments in the L coordinate is frequently termed radial diffusion because the process changes the radial distances of trapped particles from the Earth. This type of diffusion is crucial in forming the radiation belts as it provides a mechanism for transporting particles from the outer boundary of the magnetosphere into the inner belt. It also leads to the redistribution of particles injected or accelerated during magnetic storms and substorms. While radial diffusion may be overshadowed at times by the massive injections which occur during large storms and substorms, its role in bringing particles inward, in accelerating trapped particles and in redistributing newly injected particles is of major importance.

Since the third invariant Φ is proportional to L-1, radial diffusion must proceed by fluctuations in the third invariant. Variations in a trapped particle's third invariant require changes in the electric or magnetic fields that are more rapid than the particle drift frequency. Drift periods vary from tens of seconds to about a day (see Appendix B), hence, perturbations over a wide range of frequencies can alter the third invariant. Because the gyration and bounce periods are much shorter than the drift period, the first and second invariants are less likely to be affected by many of these field perturbations.