Coordinates for Representing Radiation Belt Particle Flux

Fifty years have passed since the parameter \textquotedblleftL-star\textquotedblright 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 \textquotedblleftdistance to the equatorial point of a field line,\textquotedblright McIlwain\textquoterights L-value, and the trapped particle\textquoterights 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 \textquotedblleftrecipes\textquotedblright 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.