Jupiter's magnetosphere (F. Bagenal, Ann. Rev. Earth Pl. Sci., 20, 289, 1992).
http://vega.lpl.arizona.edu/~gilda/jovianplanets.htmlThe outer planet most extensively studied has been Jupiter. Its Galilean satellite Io has the most active volcanism in the Solar System, and its atmosphere interacts electromagnetically with the surrounding medium, the Io plasma torus. This interaction is itself the source of the oxygen and sulfur ions of the torus. The plasma is picked-up by Jupiter's magnetic field into corotation with the ionosphere. Eventually, the plasma populates the entire Jovian magnetosphere. Other contributions are from the ionosphere and the solar wind. Jupiter's strong magnetic field, fast 10-hr rotation, and internal plasma sources result in an immense a magnetosphere with a strong corotational character. It is stretched and flattened by an equatorial current/plasma sheet extending from the Io/torus source at 6 Rj out to ~30 Rj where corotation breaks down. (1 Rj ~ 11 Re ~ the size of the Earth's magnetosphere!) The outermost regions are influenced by the solar wind pressure, and so the magnetopause can quickly change size by a factor of 2 following changes in the solar wind. The coupling of this giant, fast-rotating, internally supplied magnetosphere with the solar wind is complex. Even with extended Galileo in-situ measurements since 1995, we still have a lot to learn about this system. The atmosphere-magnetosphere interaction produces 10^14 Watts of auroral power, with emissions spanning from X-ray to radio wavelengths. This is 10,000 times more powerful than the Earth's aurora, and could light all cities around the world!
Emissions from both H2 and H are detected in these images. The HST images have shown that the aurora is dominated by main ovals of bright and discrete emission at high latitudes on the north and south polar regions, mapping farther than 15 Rj into the middle magnetosphere. These are accompanied, at least on the north, by emissions that are poleward of the main oval, mapping to the outer magnetosphere. The exact mapping is hindered by uncertainties in the magnetic field models, in the internal non-dipolar components and contributions from the current sheet and magnetopause currents. The STIS image clearly shows emissions at lower latitudes of the main ovals that are at the footprint of the magnetic field lines connected to Io and its extended (wake) tail. Emissions mapping to Europa and Ganymede have also been seen (Clarke et al. 2002).
One major question about this fast-rotating system is to understand the importance of dependencies of the aurora and thus of the magnetosphere on corotational properties which are fixed in magnetic longitude and thus internally controlled, versus dependencies on the magnetic local time which are dominated instead by the solar wind interaction. All the auroral emissions are highly variable in brightness and morphology, and we can use these variations to learn more about the system. The WFPC2 images shown on the left figure show a partial Jovian rotation. Some spots of emission along the ovals were seeing to corotate with the planet. However, other properties were seen to depend on the magnetic local time. In particular, the brightest emissions were seen associated with a strong auroral event that was taking place and remained confined to the dawn regions. There are other, more subtle effects that we can see in the images which we are currently studying, such as the behavior of the emissions as the rotate from the morning to the afternoon.
The Galileo orbiter has toured the system since the mid 1990s, and has provided much valuable information about the Jovian magnetosphere. On December of 2000 the Cassini spacecraft flew by Jupiter on its way to Saturn, and could measure the solar wind impinging on Jupiter and dusk side of the magnetosphere. Galileo was also making its in-situ measurements of the system. HST also took a limited set of auroral images during this period. Some results have been published from this special set of coordinated observations (Science, 2002). But a full study of that STIS dataset still awaits.
Much information is still buried in the extensive HST dataset, which started to be collected in 1994 with WFPC2. This is the basis for our archival program at the Univ. of Arizona. In addition, MHD simulations of the Jovian system and its interaction with the solar wind have only recently been made, and we are aiming at comparing the auroral observations with the MHD simulations, magnetic field models, and simultaneous Galileo measurements made by our colleagues at UCLA.