The magnetic‐anomaly model of the Jovian magnetosphere: A post‐Voyager assessment

Abstract

We reexamine the three predictions that we previously put forth as tests for the magnetic‐anomaly model (in which the anomalously weak magnetic field region in the northern hemisphere of Jupiter influences the outer Jovian magnetosphere by one or more plasma interaction processes), taking into account the Voyager and other recent observations. Concerning the prediction of a restricted longitude range of enhanced interaction between Io and Jupiter's ionosphere, the longitudinal asymmetries seen in ground‐based observations of sulfur emissions from the Io torus as well as in Voyager observations of Jovian auroral emissions agree with the predicted asymmetries. The lowered estimates of the Alfvén speed in the Io torus as a result of Voyager composition determinations modify many earlier ideas on the magnetosphere/Io interaction, but the conclusion remains that longitudinal asymmetries with a 360° period can be plausibly explained only by reference to magnetic‐anomaly effects. Concerning the predictions of plasma, energetic particle, and magnetic field periodicities in the outer magnetosphere being related to either a current sheet or a magnetic‐anomaly effect, we conclude that the hitherto common interpretation of Voyager observations in terms of a current sheet with no longitudinal asymmetries is seriously deficient because a bending of the current sheet from 40 to 60 RJ outward is required whereas the Pioneer 10 observations demand that there be no such bending; attempts to reconcile Voyager and Pioneer by invoking time variations or solar wind effects are inconsistent with the observed constancy of propagation speeds for configurational changes and with the observed ratio of magnetic pressure to solar wind pressure. We construct a simple quantitative model with a sinusoidal longitudinal variation of inverse speed and show that the inclusion of such a variation of propagation speed (as implied by the magnetic‐anomaly model) into the description of the current sheet removes these inconsistencies and allows a unified interpretation of both Voyager and Pioneer observations. Concerning the prediction of a longitudinally restricted distribution of magnetopause crossings, the evidence for a clustering of magnetopause crossings near the active sector remains suggestive but statistically inconclusive. These results, combined with a number of other observed phenomena that cannot be accounted for by a pure magnetodisc model with no longitudinal asymmetries, provide evidence that magnetic‐anomaly effects play a significant role in determining the behavior of the Jovian magnetosphere.