Periodicities in the Jovian magnetosphere: Magnetodisc models after Voyager

The Voyager-1 and 2 outbound observations of periodic double-peak flux maxima (which mark encounters with the magnetodisc) are separated into two distinct branches - a ''leading'' branch (N→S disc crossings) and a ''trailing'' branch (S→N disc crossings). On a plot of System III (1965.0) longitude vs, radial distance, the leading branch has a positiv slope (approx.1.20/R/sub J/) and the trailing branch has a relatively flat slope (approx.0.30/R/sub J/). The two branches meet at approx.80--100 R/sub J/, beyond which periodic single peak or closely-spaced multiple peaks are generally observed. This paper examines this structure using the 3 principal disc models which have emerged to explain periodicities in the Jovian magnetospheres: (1) the rigid magnetodisc model, (2) the bent or warped magnetodisc model, and (3) the wavy mangetodisc model. The rigid disc model does not allow for the merging of the double-peak structure into a single-peak structure, and the bent disc model cannot explain the phase lag of the leading peaks. Both the merging of the peaks and the phase asymmetry can be explained by a wavy-disc model in which the wave amplitude is fixed. The disc surface is then described in Jovigraphic cylindrical coordinates by z=-r0 tan α cos [phi+Ω (r-r0) /V0] where r0approx. =20 R/sub J/, αapprox. =10.70 is the dipole axis tilt angle, Ω=2/sub π//10 hr-1 is the angular speed of Jupiter's rotation, and V0approx. =40 R/sub J//hr is the speed of wave propagation. Count rate profiles along the Voyager trajectories may then be reproduced using a simple model in which the rates vary exponentially in radial distance and deltaz, the distance from the disc surface