Survey of thermal ions in the Io plasma torus with the Galileo spacecraft

Abstract

The densities, temperatures, and bulk flow velocities are reported for a series of six passages of the Galileo spacecraft through Io's torus in Jupiter's magnetosphere. These observations were gained with the plasma analyzers on December 7, 1995, and July 1–2, August 12, September 14, October 11, and November 25, all of the latter in 1999. These plasma analyzers provided high‐resolution measurements of the energy/charge (E/Q) spectra of the three‐dimensional velocity distributions of the thermal ions that included determinations of the mass/charge (M/Q) of the primary ions with a mass spectrometer. The four dominant ions in the hot torus were two populations of ions with M/Q = 16 and two smaller populations with M/Q = 8 and 32, respectively. The identification of these four ions was based upon a best fit to the E/Q spectra of the measured three‐dimensional ion velocity distributions. The two distributions with M/Q =16 were characterized by two different temperatures, in the ranges of 20 to 30 eV and 60 to 80 eV, respectively. On the basis of expectations of higher temperatures with higher masses for the pickup ions the cooler population is identified as O+, and the hotter as S++. The two smaller populations with M/Q = 8 and 32 are identified as O++ and S+, respectively. The temperatures were ∼20 eV for the O++ and 60 to 80 eV for the S+. The densities and temperatures of the ions in the hot torus remained constant during the period July through November 1999. However, the ion densities during the initial passage on December 7, 1995, were greater by factors of ∼3, which support the presence of long‐term density variations of the torus plasmas but with relatively small fluctuations in the temperatures. A complete survey of System III longitudes was acquired with the set of six passages. The presence of an “active sector” at longitudes in the approximate range of 180° to 230° as originally found with remote observations of the torus brightnesses is confirmed with these Galileo measurements. In addition, further evidence for the importance of interchange motions for radial plasma transport was also evident in several of the Galileo passages, a dynamical process which was first identified in fields and particles measurements during the first passage through the torus on December 7, 1995. A persistent lag with an average in the range of ∼2 to 4 km/s in the azimuthal flows relative to that for rigid corotation was detected and supports the previously proposed System IV coordinate system that has a slightly slower rotation rate relative to that for System III.