Periodicities in the Jovian magnetosphere: Magnetodisc models after Voyager

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Abstract
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 positive slope (∼ 1.2°/RJ) and the trailing branch has a relatively flat slope (∼ 0.3°/RJ). The two branches meet at ∼ 80‐100 RJ, 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 magnetosphere: (1) the rigid magnetodisc model, (2) the bent or warped magnetodisc model, and (3) the wavy magnetodisc 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 = −ro tan α cos [ϕ + Ω (r − ro) /Vo] where ro ≈ 20 RJ, α ≈ 10.7° is the dipole axis tilt angle, Ω = 2π/10 hr−1 is the angular speed of Jupiter's rotation, and Vo ≈ 40 RJ/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 δz, the distance from the disc surface.

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