Abstract
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 because of 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 and
abilities to model radiation belt dynamics. The core objective of this
review is to address the confusion that emerges from the coexistence of
various definitions for radial diffusion, and to highlight the
complexity and subtleties of the problem. To contextualize, we provide
an 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, to describe radiation belt drift dynamics, and to
carry contemporary radial diffusion research.