A NOTE ON VECTOR FLUX MODELS FOR RADIATION-DOSE CALCULATIONS

This paper reviews and extends modelling of anisotropic fluxes for rad iation belt protons to provide closed-form equations for vector proton fluxes and proton Bur anisotropy in terms of standard omnidirectional flux models. These equations provide a flexible alternative to the da ta-based vector flux models currently available. At higher energies, a nisotropy of trapped proton flux in the upper atmosphere depends stron gly on the variation of atmospheric density with altitude. Calculation s of proton flux anisotropies using present models require specificati on of the average atmospheric density along trapped particle trajector ies and its Variation with mirror point altitude. For an isothermal at mosphere, calculations show that in a dipole magnetic field, the scale height of this trajectory-averaged density closely approximates the s cale height of the atmosphere at the mirror point of the trapped parti cle. However, for the earth's magnetic field, the altitudes of mirror points vary for protons drifting in longitude. This results in a small increase in longitude-averaged scale heights compared to the atmosphe ric scale heights at minimum mirror point altitudes. The trajectory-av eraged scale heights are increased by about 10-20% over scale heights from standard atmosphere models for protons mirroring at altitudes les s than 500 km in the South Atlantic Anomaly, Atmospheric losses of pro tons in the geomagnetic field minimum in the South Atlantic Anomaly co ntrol proton flux anisotropies of interest for radiation studies in lo w earth orbit. Standard atmosphere models provide corrections for diur nal, seasonal and solar activity-driven variations. Thus, determinatio n of an ''equilibrium' model of trapped proton fluxes of a given energ y requires using a scale height that is time-averaged over the lifetim e of the protons. The trajectory-averaged atmospheric densities calcul ated here lead to estimates for trapped proton lifetimes. These lifeti mes provide appropriate time-averaging intervals for equilibrium model s of trapped proton fluxes.