Geophysical Research Letters

Intermittent short-duration magnetic field anomalies in the Io torus: Evidence for plasma interchange?



On Galileo's inbound pass through the Io plasma torus in December 1995, we detected intermittent, short-duration increases in the magnetic field magnitude, notable by their abrupt onset and subsequent abrupt recovery to the previous level. The events were small and brief, with an average field change of 15.7 nT (1–2% of the background field) and an average duration of ∼26 s. They were observed first shortly after data at high time resolution were first acquired at a radial distance of 7.7 RJ from Jupiter's center. The last and most clearly-defined signature, with exceptionally sharp boundaries, was observed at 6.03 RJ, about 12 minutes before closest approach to Io. The magnetic pressure change across the boundaries of these structures was ∼10 to ∼33 nPa, the latter value in a region with background plasma pressure of 60 nPa. Thus, although of small amplitude in the magnetic field, they represent large changes in the pressure. Abrupt changes in the measurements of the other fields and particles instruments correlate with some of the magnetic signatures. In this paper, we present the magnetometer data for these unusual signatures, discuss the limits on particle density changes that they imply, the significance of their spatial distribution, and their relationship to transport of plasma in the torus.