Coordinates for Representing Radiation Belt Particle Flux

Fifty years have passed since the parameter “ L‐star ” was introduced in geomagnetically trapped particle dynamics. It is thus timely to review the use of adiabatic theory in present‐day studies of the radiation belts, with the intention of helping to prevent common misinterpretations and the frequent confusion between concepts like “distance to the equatorial point of a field line,” McIlwain's L ‐value, and the trapped particle's adiabatic L* parameter. And too often do we miss in the recent literature a proper discussion of the extent to which some observed time and space signatures of particle flux could simply be due to changes in magnetospheric field, especially insofar as off‐equatorial particles are concerned. We present a brief review on the history of radiation belt parameterization, some “recipes” on how to compute adiabatic parameters, and we illustrate our points with a real event in which magnetospheric disturbance is shown to adiabatically affect the particle fluxes measured onboard the Van Allen Probes.