Fig. 5.3. A simplified view of Jupiter's offset, tilted magnetic poles, magnetic field lines, and auroral ovals. (After Rogers 1995 [193]).

form of high-energy 'radiation belts', both around Jupiter and around Earth [195]. We can imagine magnetic field lines by remembering how iron filings behave in the vicinity of a simple magnet. Remember the experiment in school? The teacher places a magnet on a table, and then pours iron filings around it. The iron filings organize themselves around the magnet in the shape of, and along the magnetic field lines of, the magnetic field running through the north and south poles of the magnet. The magnetic field lines of a planet behave and look much the same way.

The field itself probably originates deep within Jupiter. It is suspected that the field originates from 'dynamo' circulation deep inside Jupiter, within the outer shell of the hypothesized metallic hydrogen that is thought to make up the mantle of the planet. Like Jupiter, the magnetic field rotates, but with a rotation period of 9 h 55 min and 29.71 s. This rotation period is defined as System III, which is also the rotation period that coincides with radio bursts from the planet that are synchronized to particular orbital positions of the moon Io [196]. The axis of rotation of the magnetic field is tilted 9.6° from Jupiter's polar axis and it is offset 0.12 R (RJ is one Jupiter radius) from Jupiter's center. System III is not to be confused with System I and System II rotation periods.

Of course, the existence and scale of Jupiter's magnetosphere was known from Earth based radio observations. But the exact intensities of the magnetic field and particle populations, and the detailed structure and true overall extent could not be accurately discerned until the spacecraft flybys of the 1970s and 1980s. Over time, various spacecraft examining different regions of the magnetosphere have g W provided a better understanding of the magnetosphere, showing how much the

© .£ structure changes over time, and revealing the importance of the moon Io as a

C "]2 source of material (Fig. 5.4) [197]. By the end of the Pioneer/Voyager era, we knew q 3 jg the following (1) An electric current of more than a million amperes flows along

.¡5 2 the magnetic flux tube linking Jupiter and Io. (2) A doughnut-shaped torus con-

3 ^ taining sulfur and oxygen ions surrounds Jupiter at the orbit of Io. This torus u (A emits ultraviolet light, has temperatures of up to 100,000 K, and is populated by more than 1,000 electrons cm-3. A region of 'cold' (i.e., forced to rotate with the magnetic field) plasma exists between Io's orbit and the planet. It has larger than expected amounts of sulfur, sulfur dioxide, and oxygen, all probably derived from Io's volcanic eruptions. (3) The Sun-facing magnetopause (outer edge of the magnetosphere where the solar wind meets the magnetosphere) responds rapidly to changing solar wind pressure, varying from less than 50 Jupiter radii to more than 100 Jupiter radii from the planet's center. (4) A region of 'hot' (i.e., not forced to rotate with the magnetosphere) plasma exists in the outer magnetosphere. It consists primarily of hydrogen, oxygen, and sulfur ions. (5) Jupiter emits low frequency radio waves (wavelengths of one to several kilometers). The amount of radiation is strongly latitude-dependent. (6) There exists a complex interaction between the magnetosphere and the moon Ganymede. This results in deviations from a smooth magnetic field and charged particle distributions that extend up to 200,000 km from the satellite. (7) About 25 Jupiter radii behind the planet, the character of the magnetosphere changes from the 'closed' magnetic lines to an extended magnetotail without line closure. This occurs as a result of downstream interaction with the solar wind. (8) Jupiter's magnetotail extends to the orbit of Saturn - more than 700 million km 'downwind' of Jupiter [199].

For perhaps the first time, the simultaneous observations by Cassini and Galileo gave scientists a glimpse of the actual shape and curvature of the bow shock and magnetopause when the spacecraft were 100 Jovian radii apart, with Galileo sunward of Cassini and outbound from Jupiter on its 29th orbit [200]. The Cassini flyby of Jupiter offered an unprecedented opportunity, since Galileo was still in orbit about

Fig. 5.4. A simplified view of the Io Torus, the Io Flux Tube, Sodium Cloud, Plasma sheet, and Polar Aurora. (After Rogers 1995 [198]).

the planet. As Cassini approached, one spacecraft characterized the solar wind, the other sensing the response of the Jovian magnetosphere. Later, as the relative positions of the two spacecraft changed, the roles reversed. For a short time both spacecraft ntng were in the magnetosphere. The two spacecraft obtained data that showed the Jovian ein magnetosphere being compressed as a result of an inter-planetary shockwave, gen- nmnd erated by the Sun, that impinged upon it [201]. As Cassini was inbound in situ (i.e, O 3 ¡5

in the environment of Jupiter), it made measurements of the upstream solar wind irroite conditions while Galileo was deep in the magnetosphere. Later, when Cassini was ¡5

outbound moving past Jupiter, the roles reversed as Galileo was out in the solar wind ^ Ifl n and while Cassini crossed in and out of the magnetosheath [202].

Jupiter's interaction with the solar wind is not passive by any means. Its interaction is actually very active, and its magnetosphere leaves a distinct impression on the solar wind, even far upstream. The volcanoes of the moon Io supply heavy ions of mostly oxygenated sulfur to Jupiter's magnetosphere. According to Hill, once in the magnetosphere, these ions are energized and can escape magnetic confinement if they are neutralized through charge-exchange interactions. These energetic neutral atoms form a planetary wind that can actually move upstream against the solar wind. These ions are not coupled to the solar wind because they are not charged. Eventually, these energetic neutral atoms are re-ionized and do become part of the solar wind. However, because the solar wind is mainly composed of hydrogen like the Sun, the heavy ions retain the distinctive signature of their origins in the volcanoes of Io; thus, providing clear evidence of the Jovian magnetosphere's impact on the solar wind [203].

Remember that the magnetosphere is composed of the outer zone, the middle zone, and the inner zone (Fig. 5.2). The magnetic field in the outer magnetosphere is weak and variable on timescales of an hour or less, especially near the magnetopause. However, the plasma here is very rarefied and hot. Just inside the magnetopause it is the hottest thermal population known anywhere in the solar system (even the Sun), at 300-400 million Kelvin. However, the particles are very sparse. It is the pressure of this super hot plasma, and not the magnetosphere, that actually holds back the solar wind [204]. As the solar wind encounters the magnetosphere it is deflected around the planet. The magnetosphere is compressed on the sunward side, and as the solar wind slips around it, the magnetosphere is drawn out into a long tail. This magnetotail is quite long and can be 150-200 RJ. The "tail" can break off and travel "downwind" from the planet in the solar wind. Pioneer 10 and Voyager 2 encountered pieces that had broken off as far out as the vicinity of Saturn's orbit. The orbit of the Galileo spacecraft was maneuvered so that the spacecraft could explore the magne-totail, which it passed through at a distance of 143 RJ [205].

The middle zone extends from the Io plasma torus out to a distance of ~30-50 RJ. This part of the magnetosphere is dominated by the plasma sheet which is ~5 RJ thick [206]. According to Harland, the Galileo spacecraft demonstrated that the disk-like plasma sheet extends at least 100 RJ from Io's torus from which it derives, and that the thickness of the undulating disk varies significantly from one 10-h rotation to the next [207]. The plasma sheet is approximately equatorial and co-rotating although not exactly so. The plasma sheet is hot, although not nearly as hot as the hot plasma in the outer zone. Thus we refer to the plasma sheet as the 'cooler plasma.' Io and its neutral atomic cloud supply the Io plasma torus, which diffuses outward to form the plasma sheet. The composition of this denser but cooler plasma sheet is dominated by sulfur and oxygen from Io. In fact, most of the particles of the magnetosphere come from Io. The Io torus is the densest part of the magnetosphere and occupies a space starting at ~5.5 R and extending to 8 Rj [208]. By comparison, the orbital radius of Io is 5.9 Rr E W The inner zone extends from 5.5 Rj to 1.5 Rr This region is dominated by

© Jupiter's intrinsic magnetic field. Here the total plasma density decreases but the

£ "]2 highest energy particles reach their peak intensities [209]. In fact, inside the inner ig 3 ¡^ zone a human would receive a lethal dose of radiation in less than a minute.

2 To review, Jupiter's magnetosphere is basically composed of three zones. On j« ¡3 the sunward side, the outer zone starts at the magnetopause and extends inward ui Ifl n toward Jupiter to ~50 RJ. The middle zone extends from the outer zone inward to the Io torus at 8 RJ. The inner zone is dominated by the Io torus, which extends from 8 Rj to 5.5 R.

To further understand the structure of the magnetosphere, it can be helpful to examine the many parts of the magnetosphere as though we were in a spacecraft traveling to Jupiter from the direction of the Sun.

As we travel along with the solar wind, we find that we are suddenly compressed with other particles as we approach the vicinity of Jupiter. Here a bow shock develops. As we pass through the bow shock, we enter a relatively thin region called the magnetosheath. Soon, we cross an imaginary line called the magnetopause where the solar wind actually meets the magnetosphere and is deflected around and past the planet. We are now in the rarified plasma known as the outer zone. Next, we reach the equatorial, co-rotating plasma sheet of the middle zone. Finally, we reach the inner zone and the Io torus. Leaving this, we move back into the middle zone followed by the outer zone. These zones are somewhat stretched out on the side away from the Sun. And finally, we move into the magnetotail, which is the magnetosphere on the side away from the Sun that is drawn out by the solar wind. According to Rogers, the magnetotail starts ~150 Rj from the planet, where plasma breaks away from the co-rotating plasma sheet and flows down the tail [210]. Since the magnetotail has been detected all the way to the vicinity of Saturn, we see from our travel that Jupiter's magnetosphere truly is an enormous structure!

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