The daily variation of trajectory-derived high-latitude cutoff rigidities in a model magnetosphere

Abstract

Utilizing the trajectory-tracing procedure, we have calculated the vertical cutoff rigidity at various latitudes along the 260°E meridian for 8 different local times using a model of the magnetosphere represented by a sixth-degree expansion of the internal field combined with external sources due to currents in the tail and the magnetopause. We find that cutoff rigidities calculated in this magnetospheric model have a pronounced daily variation between the invariant latitudes 60° and 75°, with maximum values near noon and minimum values near midnight. At each local time, vertical cutoff rigidities calculated by means of this model are less than the values found previously by using only internal magnetic-field sources, and the lowest rigidity particles incident at any local time arrive via the magnetospheric tail. We also find that the daily variation of the proton cutoffs is asymmetric with respect to local noon, the dawn cutoff values being higher than the cutoff values for twilight. This effect is attributed to the longitudinal asymmetry of the magnetosphere. The electron daily variation curve has the inverse asymmetry. Various proton and electron experimental results are found to be consistent with these theoretical calculations.