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16. Planetary Magnetospheres
Space missions to othe planets of the solar system have shown that most of them are magnetized. In particular, the giant planets are magnetized much more strongly than Earth [Bagenal, 1992] and their magnetospheres are all much larger than ours, in part because of the stronger dipole moments, in part because the solar wind becomes increasingly rarefied far from the Sun. Tiny Mercury has a magnetic moment only about 1/2000 that of Earth and a very small magnetosphere, Venus seems non-magnetic and Mars may or may not have a weak field. The magnitudes of the dipole moments of Mercury, Earth, Jupiter, Saturn, Uranus and Neptune, in units of 10^25 Gauss-cm3, are 0.004 (approx.), 7.9, 150,000, 4300, 420 and 200, respectively [Lepping, 1995].
The most striking thing about these magnetospheres is their great diversity and this brief overview cannot possibly do justice to the extensive research done on them. Good accounts of the initial observations and of many associated discoveries can be found in special sections of the journal Science [1974, 1975a, 1975b, 1979a, 1979b, 1980, 1981, 1982, 1986, 1989, 1992], published soon after the planetary encounters by Pioneer 10 (Jupiter), Mariner 10 (Mercury), Pioneer 11 and Voyager 1 (Jupiter and Saturn), Voyager 2 (Jupiter, Saturn, Uranus and Neptune) and Ulysses (Jupiter). The Galileo spacecraft reached Jupiter in December 1995 and entered an orbit around the planet, after successfully launching a probe into Jupiter's atmosphere.
The strongest magnetic field and the most intense trapped radiation are found in the magnetosphere of Jupiter, which is also the largest [Dessler, 1983]. This was furthermore the first planetary magnetosphere to be discovered: in 1955 strange radio noise was traced by Burke and Franklin to the planet Jupiter [Franklin, 1959, 1985], although it was only attributed to magnetically trapped plasma after the discovery of the Earth's radiation belt [Drake, 1985].
Jupiter's magnetosphere is loaded with ions of sulfur and also of sodium, ejected from "volcanoes" on the satellite Io. Io also has an ionosphere with an interesting dynamo interaction with Jupiter [Ness et al., 1979]. Jupiter's trapped plasma carries a dense ring current, and seems to co-rotate with the planet, perhaps up to the magnetopause. Its density profile contains dips due to absorption by Jupiter's moons and by the planet's thin ring, which resembles Saturn's ring but is much narrower; the existence of that ring was first suggested by an absorption feature in the belt [Acuna and Ness, 1976]. Jupiter also has an aurora, observable from Earth, and radio emissions with complicated patterns, some of them correlated with the position of Io.
Saturn's magntosphere similarly tends to rotate with the planet and contains absorption features. The planet seems to have an inner belt like the Earth's, believed to arise from albedo neutrons knocked out of the planet's rings by cosmic rays [Cooper and Simpson, 1980].
The Earth's magnetic axis is very close to its rotation axis. Similar proximity between the two axes was found for Jupiter, Saturn and Mercury (for Saturn the axes coincided within observational error), and this was therefore widely held to be a general feature of planetary magnetic fields. At the time of the encounter between Voyager 2 and Uranus, on 24 January 1986, the planet's axis pointed within a few degrees of the Sun. It was therefore expected that here was a "pole-on" magnetosphere, a previously unstudied configuration in which the axis of the planetary magnet pointed approximately into the solar wind.
But it was not to be. The magnetic axis of Uranus--and later also that of Neptune--was found to make an angle of about 60 degrees with the planetary rotation axis, causing the field to swing widely with each rotation of the planet. As Uranus orbits the Sun, there will arise occasions when a "pole-on" magnetosphere is (briefly!) realized, but it did not happen during the Voyager 2 encounter.
Finally, Mercury's magnetosphere [Ness, 1979] seems to be too small for energetic particles to become trapped in it. However, as Mariner 10 went past the planet's night side, it encountered a burst of energetic particles, which could be the result of a substorm-type event in Marcury's magnetic tail.
Interesting magnetic cavities are also formed around Venus, the Moon and comets (and probably, Mars), but if the obstacle is not a planetary magnetic field, the cavity produced is quite different from the ones described above. All this suggests a rather rich field for future research, involving configurations unlike the Earth's, on which many additional observations still remain to be made.
17. Other Areas A brief overview like this one must by necessity omit many important topics, such as:
18. Assessment
The preceding brief history only covers scientific aspects of magnetospheric physics. In addition, magnetospheric physics also has institutional, personal and social aspects.
An institutional history traces the evolution of the field and its accomplishments in the framework of the organizations which led it, of institutions, committees, executive decisions and of the individuals involved in them [e.g. Ezell, 1988]. An instructive example is "Beyond the Atmosphere" [Newell, 1980], an account of NASA's effort in space science 1958-1975 by a former NASA Associate Administrator who led those efforts for many years. It covers all fields, not just magnetospheric physics, but where its subject overlaps this narrative, it often paints a strikingly different picture.
Personal histories are first-hand accounts by participants. At best they give an unequalled intimate view of the discovery process. At worst they are carefully filtered, and their writers also do not always have the necessary discrimination and writing skill. Such deficiencies would matter less if such accounts were plentiful enough to allow comparison and cross-checking: sadly, only very few exist, which makes them particularly valuable, and their coverage of the field is rather patchy [Van Allen, 1983a, 1990 ; Eather, 1980, chapt. 19; Frank, 1990; Gombosi et al., 1994].
The community of magnetospheric physics has never been properly studied. It is relatively small: the membership of AGU's Section on Space Physics and Aeronomy stands around 3000 (1980--1604; 1985--1922; 1990--about 2600). This also includes scientists whose main interests are the upper atmosphere, interplanetary space and the Sun, but on the other hand may miss many workers outside the USA. As noted, this discipline arose from three main sources--plasma physics, work with rockets, balloons and ground instruments, and the study of cosmic rays. It began assuming its separate identity in 1959, when (led by James Van Allen) it chose the American Geophysical Union (AGU) as its home organization and the Journal of Geophysics Research (JGR) as its main means of communication.
Today that community is in a serious crisis, made evident, for instance, by a frustrating slow-down in the rate of discovery during the last decade 1984-1994. It may be instructive to speculate about the causes of this slow-down and its implications to the community's future.
One can roughly divide the record of magnetospheric physics in the space age into three periods: (1) the era of discovery, 1958-1965; (2) the expansion stage, 1965-1977; and (3) the era of stagnation, setting in gradually after 1977.
In the first period, the large-scale morphology was surveyed--particle populations, the main regions and the boundaries. In addition, this was the beginning of our ideas on convection and reconnection.
In the expansion stage, details were filled in--correlations with the IMF, substorm morphology, Birkeland currents, E//, auroral kilometric radiation, O+ ions in the ring current, ion beams and conics, injections at synchronous orbits, etc. Additional theoretical ideas were also introduced--the NENL theory of substorms, the Brice-Nishida theory, the Coroniti-Kennel theory, theories on the consequences of convection by Schield et al. and by Vasyliunas, and others not touched on here.
Since 1977 some observational details were added., e.g., about the magnetosphere with IMF Bz > 0, about the distant tail (by ISEE-3 and Geotail), the ring current (by AMPTE-CCE) and the plasma sheet (by ISEE 1-2 and AMPTE-IRM). Theories, too, have improved, but the main problems continue to elude us--the nature of substorms, structure of the open magnetopause, specifics of reconnection, convection in the tail, global structure during northward IMF and similar questions.
Why this apparent pause? Three possible reasons will be noted here: the nature of discovery, the choice of mission strategy and a missed transition in the evolution of magnetospheric physics.
(1) There exist two kinds of discovery in this field--discovery of new problems and discovery of solutions. The heady early period seemed packed with discoveries, but most of them belonged to the first kind. It was inevitable that satelites passing for the first time through the radiation belt, the magnetopause, cusp, bow shock or plasma sheet would make an important discovery; but while new phenomena accumulated, explanations of their features lagged, and they still do. In laboratory physics, when a new phenomenon is discovered, one can design experiments to focus on it; but magnetospheric physics, in common with the rest of geophysics, offers few controlled experiments and depends primarily on observations. Thus progress towards explanations is slow and uncertain.
(2) The cost of spacecraft is high, both in funds and efforts. All early space missions therefore involved isolated spacecraft, but it seems that the amount of information available from this mode is just about exhausted. Magnetospheric physics is synergistic: to understand global behavior, a coordinated network of satellites is needed.
After 1977 the field was ripe for such a network, but unfortunately the use of isolated spacecraft is still the norm. The Russian Interball (two spacecraft launched in 1995) and the European Cluster (due in 1996) each contain four coordinated spacecraft and promise to give valuable results, in particular in conjunction with the "Wind" and "Polar" spacecraft of the US. But a meaningful coverage demands a much larger number of platforms, as was made clear by CDAWs, Coordinated Data Analysis Workshops [e.g. Manka et al., 1982], which tried to analyze specific events and generally found that even with all available data, important questions could not be resolved.
(3) As noted, the magnetospheric community first assumed a separate identity around 1960. Independent space physics departments were established at selected universities--Iowa, UCLA, Rice, Alaska, then more--and space research groups were set up at NASA, Johns Hopkins Applied Physics Lab., Los Alamos etc. As the community expanded in 1965-77, it also began raising its first generation of internally-trained scientists.
But something seemed missing. A community needs not only its institutional identity but also a core of its accumulated knowledge, set up in an orderly way that can be passed on. Research and symposia lead to review talks and papers, which in turn lead to textbooks and courses, telling "such-and-such we are pretty sure of and can teach, this-or-that is unclear or controversial, and here are the boundaries of our knowledge."
Even now, rather little of this process of distillation has taken place, especially in observations: in substorm morphology, for instance, there is surprisingly little that can be regarded well-established. Possible reasons are too long and too controversial to list here, but the result has been a narrowness of scope and a lack of broad vision which even now hamper further progress and further planning.
This review is altogether too short to properly describe what such "core knowledge" may contain. Still, one hopes it will give its readers, especially younger members of the community, a uniform historical framework of the overall structure of their field.
Acknowledgments
A draft of this article was given for review to a large number of scientists involved in the discoveries described here, most of whom provided useful comments. The author thanks A. Dessler, J. Heppner, P. Hart, W. Hess, R. Hoffman, E. Hones Jr., G. Ludwig, M. Peredo, H. Petschek, J. Slavin, E. Smith, J. Van Allen, M. Walt and R. Wolf for their help in compiling this review, as well as two editors and three referees of the journal.
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