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.

The magnetosphere of Neptune: Hot plasmas and energetic particles

“…Also shown on these trajectory plots are 1-hour UT time ticks along the trajectories, vertical dashed lines that show the "average" minimum L shell positions of the seven satellites (Triton and 1989N1 Table 2]). The Uranus encounter showed that the minimum L value of a satellite is where particle features are expected [Mauk et ah, 1987;Stone et al, 1986;Acuna et al, 1988]. The situation is not yet clear for rings [e.g., Paranicas and Cheng, this issue (a)].…”

Uranus' magnetic field and particle drifts in its inner magnetosphere

“…Also, a surprisingly large tilt (60 ø to the rotation axis) and offset (0.33 R t: from the planetary center) of the magnetic dipole axis were found along with an almost Sun-planet aligned rotation axis. The large tilted dipole has consequences such as large traversals of the Uranian moons over a wide range of magnetic latitudes and longitudes, resulting in peculiar particle absorption signatures [Mauk et al, 1987;Acuna et al, 1988]. The near Sun-planet aligned rotation axis leads to the speculation of a convection-dominated Uranian magnetosphere at the epoch during the V2 visit [Selesnick and Richardson, 1986;l/asyliunas, 1987].…”

“…Nevertheless, the effect of using only the first two terms to calculate the predicted locations for absorption of energetic charged particles by the Uranian moons is significant, and the magnetic equator derived from the 03 model is warped and noticeably distorted from the plane surface associated with the OTD model lacuna et al., 1988]. The 03 model, however, cannot account for some significant discrepancies in the location of charged particle absorption signatures associated with the sweeping effects of the Uranian satellites [Acuna et al, 1988], nor can it explain why some H 2 band emissions fall partly outside the 03 auroral zone [Herbert and Sandel, 1994].…”

“…Also, it was found that the octupole changes the shape of the polar cap and alters the plasma 20,257 field configuration, the L shell is often used. Because of the high nondipolar field, it is difficult to obtain a simple definition of L, and various definitions were adopted [e.g., Acuna et al, 1988;Herbert and Sandel, 1994]. In this paper, we use the Euler potentials, or a and/3 coordinates, to define the coordinates of the magnetic field.…”

Electron signatures of satellite sweeping in the magnetosphere of Uranus

“…The fact that maximum intensity was observed 7 min after reaching the Q3 minimum L is consistent with an expected latitudinal gradient (see section 5). However, the L value at the flux maximum is only 0.01 greater than that at minimum L. This observation provides a one-point test of Q3 values for L which were calculated with the approximation that drift shell splitting is negligible [Acufia et al, 1988]. Since the effects of high-order field components decline at larger distances from Uranus, we assume that the accuracy of drift shell positions calculated from Q3 is better than AL --• 0.01 in the macrosignature regions where we localize positions of intensity minima in L.…”