Nonadiabatic particle motion in the magnetosphere

Using calculations in a model geomagnetic fleld, the influence of nonadiabatic particle motion on the polar cap structure of polar protons is investigated. Breakdown of the first invariant, due to excessive field curvature and gradients near the last closed field line, results in an effect that cannot be differentiated from strong pitch angle scattering using low-altitude detectors, although no time variations of the magnetic field are considered. The results obtalned thus form a basis to which wave-particle interactions and other scattenng effects must be added. In a companion paper, these results are applied to observations of solar protons over the polar caps, and it is shown that the quiettime behavior of these particles is indeed approximately given by the constraints due to the spatial variations of the geomagnetic field.