The Io Cloud and Torus

The core of the magnetosphere, its densest part, and the source of most of its plasma, is the Io torus [211]. Indeed, Io itself plays a significant role in Jupiter's magnetosphere, as it is believed that most of the particles in the magnetosphere come from Io. Lou Frank of the University of Iowa says the Io torus is the "beating heart of the Jovian Magnetosphere [212]." Material is transported from Io to the torus in neutral atomic form via a cloud of neutral atoms that is localized around Io itself [213].

This sodium cloud surrounding Io was actually discovered from Earth by Brown in 1972, being detected by the optical emission from the sodium atoms in it. Other atoms are also present in the cloud and these atoms come from Io [214]. It was previously known that the Io torus contained sulfur, oxygen, sodium, and potassium. In 1999, astronomers at Kitt Peak National Observatory in Arizona announced that the torus also contained chlorine [215]. The Io atomic cloud is localized around Io and is generally elongated in a shape resembling that of a banana (Fig. 5.4) [216]. While Io and the atomic cloud orbit Jupiter at 17 km s-1, the plasma sheet co-rotates at 74 km s-1, overtaking the cloud. Of course the atoms in the cloud are also in orbit around Jupiter. Those that diffuse inward toward Jupiter orbit faster and those that diffuse outward orbit slower. As the co-rotating plasma overtakes the cloud, impacts cause atoms to split into ions and electrons that can j- ®

then be captured by the plasma sheet where they eventually migrate out into the c -n magnetosphere [217]. j| j-

if you imagine a giant tire inner tube encircling Jupiter's waist, or perhaps a O 3 ¡5

really big doughnut, you get a pretty good picture of the Io plasma torus. The 2

torus resides at 5-8 RJ, co-rotating with the magnetic field. Io passes through the ¡5

torus only twice per orbit around Jupiter, since the torus is tilted 7° to the orbit ^ Ifl n of Io [218]. The torus is not a physically hard, impenetrable structure. It is not to be confused with Jupiter's gossamer rings. Rather it is a torus of ionized gas, i.e., a plasma, a halo that extends all the way around Io's orbit [219]. Pioneer 10 discovered the torus in 1973 when the spacecraft detected its ultraviolet emission. Later in 1979, Voyager determined the full extent of the torus [220]. The Pioneer 10, Voyager 1, and Ulysses spacecraft have all traveled through the torus. More recently the Galileo spacecraft has penetrated the torus. As a result, we know there are three concentric parts of the torus. There is a cold, inner torus at 5.0-5.6 Rj, inside Io's orbit. It receives only 2% of the plasma from Io [221]. There is the torus peak at 5.7 RJ, just inside the orbit of Io. This is the sharp boundary between the cold inner torus and the warm outer torus [222]. And there is the warm outer torus that resides at 5.9-7.5 RJ, at and outside Io's orbit. The outer torus is the main

Fig. 5.5. Active volcanic plumes on Io. This Galileo spacecraft image shows two volcanic plumes on Io. One plume was captured on the bright limb of the moon over a caldera named Pillan Patera. This plume is 140 km (86 miles) high. The second plume is seen near the terminator (boundary between night and day) and is called Prometheus. The shadow of the 75 km (45 mile) high airborne plume can be seen extending to the right of the eruption vent. The vent is near the center of the bright and dark rings. North is toward the top of the image. (Credit: NASA/JPL-Caltech).

Fig. 5.5. Active volcanic plumes on Io. This Galileo spacecraft image shows two volcanic plumes on Io. One plume was captured on the bright limb of the moon over a caldera named Pillan Patera. This plume is 140 km (86 miles) high. The second plume is seen near the terminator (boundary between night and day) and is called Prometheus. The shadow of the 75 km (45 mile) high airborne plume can be seen extending to the right of the eruption vent. The vent is near the center of the bright and dark rings. North is toward the top of the image. (Credit: NASA/JPL-Caltech).

pool of plasma that gradually diffuses outward. As it does, it gains energy from the magnetic field and from the hot plasma that surrounds it [223]. g W There is a distinct outer boundary to the plasma torus where it comes up against

© .£ the plasma sheet of the middle magnetosphere. From 7.5-9 R the plasma density

C "]2 falls off sharply and the temperature increases dramatically, fivefold [224].

q 3 jg We do not fully understand all the mechanisms that maintain the torus. It had

.¡5 2 been thought that neutral atoms were sputtering from the surface crust of Io as they

3 ^ were being bombarded by charged particles circulating in Jupiter's magnetosphere.

u (A But the more likely process became evident only when volcanic plumes were spotted on Io's limb (Fig. 5.5). The Voyager spacecraft saw ultraviolet emission from ionized oxygen and from both single and doubly ionized sulfur in the torus. Apparently, the plasma density is related to Io's volcanic activity [225]. By the time the Galileo spacecraft was into its mission, scientists had concluded that this is in fact the case. The Jovian magnetic field snatches away and then progressively accelerates away the ionized material ejected from Io [226].

Although the low escape velocity on Io allows the volcanic plumes to rise to several hundred kilometers in altitude, the eruptions do not occur with enough velocity to launch particles into Jupiter's magnetosphere. Instead, particles in the plumes become positively or negatively charged, making them susceptible to Jupiter's magnetic field, which grabs them into the magnetosphere. These particles form the doughnut shaped torus [227].

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