On Jovian plasma sheet structure

Abstract

We evaluate several models of Jovian plasma sheet structure by determining how well they organize several aspects of the observed Voyager 2 magnetic field characteristics as a function of Jovicentric radial distance. The present study focuses exclusively on the data from the Voyager 2 spacecraft because the magnetosphere was more stable during that flyby than it was during the Pioneer 10 and Voyager 1 flybys. It is shown that in the local time sector of the Voyager 2 outbound pass (near 0300 LT) the published hinged‐magnetodisc models with wave (i.e., models corrected for finite wave velocity effects) are more successful than the published magnetic anomaly model in predicting locations of current sheet crossings. We also consider the boundary between the plasma sheet and the magnetotail lobe which is expected to vary slowly with radial distance. We use this boundary location as a further test of the models of the magnetotail. The plasma‐sheet‐lobe boundary is often identified from the observed field gradients, the gradients in the lobes being much smaller than those in the plasma sheet. We show that the compressional MHD waves have much smaller amplitude in the lobes than in the plasma sheet and use this criterion to refine the identification of the plasma‐sheet‐lobe boundary. When the locations of crossings into and out of the lobes are examined, it becomes evident that the magnetic‐anomaly model yields a flaring plasma sheet with a halfwidth of ∼3 RJ at a radial distance of 20 RJ and ∼12 RJ at a radial distance of 100 RJ. The hinged‐magnetodisc models with wave, on the other hand, predict a halfwidth of ∼3.5 RJ independent of distance beyond 20 RJ. New optimized versions of the two models locate both the current sheet crossings and lobe encounters equally successfully. The optimized hinged‐magnetodisc model suggests that the wave velocity decreases with increasing radial distance. The optimized magnetic anomaly model yields lower velocity contrast than the model of Vasyliunas and Dessler (1981). The hinged‐magnetodisc models are shown to satisfy the constraints on the plasma sheet thickness imposed by MHD theory. The magnetic anomaly model can be made consistent with the expectations from MHD theory only by assuming plasma anisotropy so large that the plasma would be unstable to mirror mode instability. We recognize that the magnetic anomaly model is a comprehensive model designed to account for many features of the Jovian system as observed in multiple flyby missions. The plasma sheet structure for the Voyager 2 epoch is only one aspect of the model's predictions. Nonetheless, we believe that the model's failure to satisfy constraints on plasma sheet structure imposed by MHD equilibrium theory presents a challenge to the magnetic anomaly model as it is currently described.