A theoretical recipe for radial diffusion coefficient computation

What is it about?

Radiation belts are most commonly described in terms of adiabatic coordinates. Each of the 3 adiabatic coordinates relates to the amplitude of motion in a frequency range: from fast gyrations around the magnetic field direction (1st coordinate: M) to slow drift motions around the Earth (3rd coordinate: L*). It is traditionally assumed that radiation belt behavior along the L* axis is set by a diffusion equation. However, it is still difficult to estimate precisely the coefficient that characterizes the intensity of the diffusion in L*, also called radial diffusion coefficient. This article describes the electromagnetic conditions that are required in order to have a variation of the L* coordinate. Then, it relates the findings to the most general topic that is radial diffusion.

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Why is it important?

Radial diffusion is a very old research topic (almost as old as the discovery of radiation belts!). Therefore, the recipe for "radial diffusion coefficient computing" has suffered from a lot of successive approximations along the way. The objective of this article is to start from scratch, and to reduce the amount of assumptions to its smallest, in order to offer the soundest evaluation of the radial diffusion process.