Implications of the GSFC Q3 model for trapped particle motion

Abstract

The adiabatic motion of energetic charged particles in the Uranian magnetosphere is studied by numerical computation of field geometric invariants associated with the GSFC Q3 magnetic field model. Comparison of the computed L shell values along the Voyager 2 trajectory with those associated with the Uranian satellites Ariel, Umbriel, and Miranda as a function of their orbital position allows the prediction of times and locations where absorption effects of charged particles should be observable. Systematic differences in the observed and predicted counting rate (flux) minima suggest that the “age” of an absorption event must be taken into account and that a better understanding of the particles’ drift history is needed to explain the observed features. As a consequence of the large tilt of the dipole term with respect to the rotation axis, the Uranian moons sweep a large range of L shells during their orbital motion and occupy several different L minima defined in the text. Comparisons with previous results obtained from the offset tilted dipole model of Ness et al. (1986) yield a complex picture regarding the predicted and observed times for absorption signatures, depending on the minimum L value used for comparison. In spite of this complexity, a general pattern is evident in which the predicted locations for the absorption signatures fall inside (i.e., at smaller radial distances) the observed features, thus suggesting that radial diffusion effects as well as the drift history of the trapped particles must be taken into account to explain the energetic charged particle observations. It is thus expected that observed absorption signatures which are “young” with respect to the location of the absorbing body and the Voyager 2 spacecraft better reflect the geometry of the magnetic field.