The Io Flux Tube and Magnetic Footprints on Jupiter

As we have discussed, Jupiter and its moon, Io, are strongly linked in an electromagnetic fashion. One of the examples of this connection is a phenomenon called "the Io flux-tube." Io is electrically linked to Jupiter by a pair of "flux-tubes" that run down the magnetic field lines in Jupiter's polar regions [278].

The Io flux tube is a fascinating feature of Jupiter's electromagnetic environment and John Rogers explains it very well. According to Rogers, "As the plasma-filled magnetic field sweeps past Io, it induces an electrical potential across Io which ntng drives an electric current, either through its body, or more likely, through its ein ionosphere. Plasma sweeping past Io tends to be entrained to the orbital speed j| j-

of Io and accelerated north or south into this current, which becomes a so-called O 3 ¡5

'flux tube' of particles running along the magnetic field lines that intersect Io. 2

The flux tube is actually guided by a wave that propagates from Io along the nvurup field lines, called an 'Alfven wave'. On the side of the flux tube toward Jupiter, u Ifl n electrons stream away from Io toward the ionosphere, and ions stream in the opposite direction. On the side away from Jupiter, the directions are reversed" (Fig. 5.4) [279].

Io also leaves a magnetic footprint on Jupiter's upper atmosphere. This footprint appears as a spot of ultraviolet emission that remains fixed underneath Io's position as Jupiter rotates. We know that the magnetic footprint of Io extends much further than the immediate vicinity of the Io flux tube interaction with Jupiter [280]. There is also faint, persistent, far-ultraviolet emission from the footprints of Ganymede and Europa. The emissions of the magnetic footprints of Io, Ganymede, and Europa were detected in ultraviolet images taken with the HST Space Telescope Imaging Spectrograph (STIS). The fact that these footprints are associated with the respective Jovian moons was established by observing that the footprint remains stationary under its moon as Jupiter rotates. Ganymede's footprint appears distinctly brighter that Europa's, and Io's is the brightest of all [281, 282].

The emission of each footprint persists for several hours downstream. That is, the ultraviolet emission signature gives the appearance of a comet, with the footprint stationary under its moon being the head, and the fading downstream emission being the tail [283]. Harland describes the auroral footprints as 'having comet-like tails' because the charged particles continue to excite Jupiter's atmosphere for some time after Io has passed overhead. Of course, we understand that it is Jupiter that rotates so rapidly under the moon as the moon slowly orbits (Fig. 5.7) [284].

There is a tremendous amount of energy in the Io flux-tubes. On December 7, 1995, the Galileo spacecraft flew past Io on its way into orbit around Jupiter. As it passed Io, the spacecraft detected bi-directional electron flows aligned with the Jovian magnetic field lines. This was the first in situ observation of the Io flux tubes. The flow of particles was equivalent to an electric current of several millions amps and an overall deposit of energy into Jupiter's atmosphere of a trillion watts; the most powerful direct current in the solar system! That is incredible energy of which the human mind can hardly comprehend! This energy generated the auroral emissions that appear as footprints in the ultraviolet, as it strikes the Jovian atmosphere [285]. "Io's downstream emission extends for at least 100° in longitude along the magnetic footprint of Io's orbit". Clarke et al., believe this implies active processes that persist for a few hours after Jupiter's magnetic field has swept past Io. Clarke et al., conclude that these downstream emissions are produced by high-energy charged particles that precipitate into Jupiter's atmosphere from the plasma torus downstream from Io, a process that continues at a declining rate for several hours after Io has passed [286]. These footprints are evident at both Jovian poles, as is the auroral polar hoods, discussed previously. The exact cause of these emissions is not fully understood.

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