Galileo radio occultation measurements of Io's ionosphere and plasma wake

Six radio occultation experiments were conducted with the Galileo orbiter in 1997, yielding detailed measurements of the distribution and motion of plasma surrounding Io. This distribution has two components. One is highly asymmetric, consisting of a wake or tail that appears only on the downstream side and extends to distances as large as 10 Io radii. The other resembles a bound ionosphere and is present within a few hundred kilometers of Io's surface throughout the upstream and downstream hemispheres. Motion of plasma within the wake was measured through cross correlation of data acquired simultaneously at two widely separated terrestrial antennas. Plasma near Io's equatorial plane is moving away from Io in the downstream direction. Its speed increases from 30 km s(-1) at a distance of 3 Io radii from the center of Io to 57 km s(-1) at 7 10 radii. The latter corresponds to corotation with Jupiter's magnetic field, which suggests that bulk plasma motion rather than wave motion is being observed. Results for the bound ionosphere include vertical profiles of electron density at 10 locations near Io's terminator. The ionosphere is substantial, with the peak density exceeding 50,000 cm(-3) at 9 out of 10 locations and reaching a maximum of 277,000 cm The peak density varies systematically with Io longitude, with maxima near the center of the hemispheres facing toward (0 degrees W) and away from (180 degrees W) Jupiter and minima near the center of the downstream (90 degrees W) and upstream (270 degrees W) hemispheres. This pattern may be related to the Alfvenic current system induced by Io's motion through magnetospheric plasma. The vertical extent of the bound ionosphere increases from similar to 200 km near the center of the upstream hemisphere to similar to 400 km near the boundary between the leading and trailing hemispheres. There is a close resemblance between one ionospheric profile and a Chapman layer, and the topside scale height implies a plasma temperature of 202 +/- 14 K if Na+ is the principal ion. Two intense volcanic hot spots, Kanehekili and 9606A, may be influencing the atmospheric structure at this location.