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

Six radio occultation experiments were conducted with the Galileo orbiter i n 1997, yielding detailed measurements of the distribution and motion of pl asma surrounding Io. This distribution has two components. One is highly as ymmetric, 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 s urface throughout the upstream and downstream hemispheres. Motion of plasma within the wake was measured through cross correlation of data acquired si multaneously at two widely separated terrestrial antennas. Plasma near Io's equatorial plane is moving away from Io in the downstream direction. Its s peed 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 w ith Jupiter's magnetic field, which suggests that bulk plasma motion rather than wave motion is being observed. Results for the bound ionosphere inclu de vertical profiles of electron density at 10 locations near Io's terminat or. The ionosphere is substantial, with the peak density exceeding 50,000 c m(-3) at 9 out of 10 locations and reaching a maximum of 277,000 cm The pea k density varies systematically with Io longitude, with maxima near the cen ter of the hemispheres facing toward (0 degrees W) and away from (180 degre es W) Jupiter and minima near the center of the downstream (90 degrees W) a nd upstream (270 degrees W) hemispheres. This pattern may be related to the Alfvenic current system induced by Io's motion through magnetospheric plas ma. The vertical extent of the bound ionosphere increases from similar to 2 00 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 t opside scale height implies a plasma temperature of 202 +/- 14 K if Na+ is the principal ion. Two intense volcanic hot spots, Kanehekili and 9606A, ma y be influencing the atmospheric structure at this location.