Direct plasma measurements in the Io torus and inner magnetosphere of Jupiter

Abstract

A model of the Io plasma torus has been constructed using the in situ plasma measurements of Voyager 1. A sharp gradient in plasma temperature of ∼7 × 10 5 K R J −1 at 5.7 R J divides the torus into two parts, a cold inner region, where the ions are closely confined to the centrifugal equator, and a warm outer region, which includes the orbit of Io and has a thickness scale height of 1 R J . The outer edge of the warm torus is defined by a drop in plasma density near 7.5 R J . The bulk motion of the plasma, i.e., the average velocity vector, is within 1% of the value expected on the basis of strict corotation in the inner part of the torus but probably deviates by 5 to 10% from corotation outside the torus. This breakdown from corotation may occur at the outer boundary of the warm torus. The energy per charge spectra show well‐resolved peaks in the inner part of the torus but strongly overlapping peaks in the outer part. In the inner torus there is a significant variation in the abundances of different ionic species over spatial scales <10 4 km. However, in the plasma sheet of the middle magnetosphere the ionic composition appears to be uniform from 12 to 42 R J and is strongly dominated by ions with a ratio of atomic mass to charge of 16. These ions are most probably some combination of O + and S 2+ ions. One consequence of the observation is that the Alfven speed is uniformly low in the outer part of the torus, with values less than 250 km s −1 .