The year 2019 marks the 60th anniversary of the concept of radial
diffusion in magnetospheric research. This makes it one of the oldest
research topics in radiation belt science. While first introduced to
account for the existence of the Earth’s outer belt, radial diffusion is
now applied to the radiation belts of all strongly magnetized planets.
But for all its study and application, radial diffusion remains an
elusive process. As the theoretical picture evolved over time, so, too,
did the definitions of various related concepts, such as the notion of
radial transport. Whether data is scarce or not, doubts in the efficacy
of the process remain due to the use of various unchecked assumptions.
As a result, quantifying radial diffusion still represents a major
challenge to tackle in order to advance our understanding of and ability
to model radiation belt dynamics. The core objective of this review is
to address the confusion that emerges from the coexistence of various
definitions of radial diffusion, and to highlight the complexity and
subtleties of the problem. To contextualize, we provide a historical
perspective on radial diffusion research: why and how the concept of
radial diffusion was introduced at Earth, how it evolved, and how it was
transposed to the radiation belts of the giant planets. Then, we discuss
the necessary theoretical tools to unify the evolving image of radial
diffusion, describe radiation belt drift dynamics, and carry out
contemporary radial diffusion research.