The study of Jupiter's magnetosphere can help us understand our own. It also raises an intriguing question: Can a planet be a pulsar?
by T. W. Hill, Rice University, Houston, Texas; and A. J. Dessler, University of Arizona, Tucson
The space between the planets is filled with a solar wind of ionized gas (plasma) expanding supersonically outward from the Sun. Most planets, including Earth and Jupiter, have an intrinsic magnetic field that deflects the solar wind, creating a comet-shaped cavity called a magnetosphere. Earth's magnetosphere, for example, extends about 10 Earth radii in the upstream, sunward direction and hundreds of Earth radii downstream. Frictional coupling at the magnetopause drives plasma circulation, known as convection, within the magnetosphere, a dynamic process that gives rise, among other things, to visible aurorae and planetary radio emissions.
Fig. 1. A sketch of Jupiter's magnetosphere, showing its cross section in the noon-midnight meridian plane. The solar wind, arriving from the left, is deflected by the bow shock to flow around the magnetopause. Jupiter's magnetic field lines, confined within the magnetopause, are stretched outward by centrifugal force and internal plasma pressure. Most of this plasma is produced in the Io torus, a ring-shaped structure near the equatorial plane at a distance of about six Jupiter radii.
Jupiter, the largest planet, also has the largest magnetosphere; it extends as far as 100 Jupiter radii (1100 Earth radii) on the sunward side (Figure 1). If it were visible, Jupiter's magnetosphere would appear larger than the Sun or Moon in spite of its greater distance from Earth. It is also the most powerful magnetosphere: its auroral and radio emissions dwarf those of all other planets combined. Jupiter's radio emissions are detectable with modest amateur equipment; its auroral emissions occur at ultraviolet and infrared wavelengths and thus require sophisticated detectors in space for the ultraviolet or at least on a high mountain top for the infrared. Jupiter's magnetosphere is powerful not only because of its size, but also because of its rapid rotation and because of the internal plasma source provided by Io, the innermost of the Galilean satellites.
The magnetosphere tends to corotate with the planet because it is coupled electrodynamically to the ionosphere. Jupiter, with a 10-hour day, is the fastest rotator among the planets. This rapid spin, when imposed on the magnetosphere, provides a vast source of power for magnetospheric phenomena. In order to tap this power source, however, the magnetosphere also needs an internal source of plasma, and this is conveniently provided by Io. Orbiting at a distance of six Jupiter radii, deep within the magnetosphere, Io is the most volcanically active known body. Its volcanic emissions, discovered by the Voyager 1 spacecraft and now monitored with Earth-based telescopes, are rich in sulfur dioxide. Through a process that is not yet completely understood, some of these volcanic emissions escape Io's gravity to produce a doughnut-shaped plasma torus of sulfur and oxygen ions in the magnetosphere near Io's orbit (Figure 1), which is observable with Earth-based telescopes (Figure 2). This torus is dynamically unstable because the outward centrifugal force due to corotation greatly exceeds the inward force of Jupiter's gravity. This instability drives a plasma outflow from the torus that must ultimately escape the magnetosphere in a planetary wind. Although it is agreed that such an outflow is inevitable, the detailed nature of this outflow is unknown and is, therefore, the subject of lively theoretical debates. Observations obtained by the Galileo spacecraft may settle some of these debates.
Fig. 2. A series of six ground-based telescopic images of the Io torus, showing emissions from singly ionized sulfur. In each of the vertically stacked images, Jupiter is at the center and the Io torus is the faint ring extending to the orbit of Io at 5.9 Jupiter radii as indicated by vertical dashed lines. The large, overexposed spots are Galilean satellites intruding into the field of view. The sequence spans one complete 10-hour rotation of Jupiter. (Courtesy of Nick Schneider, University of Colorado, and John Trauger, Jet Propulsion Laboratory.)
The most intriguing aspect of Jupiter's magnetosphere is its pulsar behavior, that is, the rotational modulation of various remotely observed magnetospheric emissions. Indeed, the first detection of Jupiter's magnetosphere was in the form of cosmic radio signals modulated at Jupiter's rotation frequency 2 decades before the first in situ measurements by the Pioneer 10 spacecraft. Most of the remotely observable emissions from the Jovian magnetosphere are modulated at the 10-hour rotation period, which is the defining property of a pulsar in the astrophysical sense. These signals include radio emissions from MHz to GHz frequencies, auroral emissions at ultraviolet and infrared frequencies, and optical emissions from the Io torus, as well as relativistic electrons ejected from Jupiter's magnetosphere into interplanetary space.
The ultimate cause of all these rotational modulations is probably the longitudinal asymmetry of Jupiter's magnetic field. Jupiter's surface field, like Earth's, only more so, has large magnetic anomalies that are superimposed on the overall dipole field pattern. These anomalies produce longitudinal variations in the electric current system that links Jupiter to its magnetosphere, and perhaps also longitudinal variations of the rate of injection of new plasma into the Io torus. Earth-based images (Figure 2, for example) show that the torus is highly variable and always asymmetric, being brightest, on average, in a particular sector of Jovian longitude called the active sector. The pulsar modulations can all be explained, in principle, if we assume that this active sector structure persists in the outflow and planetary wind. The ongoing study of this outflow process is important not only for what it will tell us about Jupiter's magnetosphere, but also for what it may tell us about the workings of the "real" pulsars that lie far beyond the reach of spacecraft measurements.
The famous fragmented comet Shoemaker-Levy 9 that collided with Jupiter and painted its atmosphere in July 1994 also left its mark on the magnetosphere. Unusual auroral emissions were observed to accompany some of the fragment impacts, and the synchrotron emission from relativistic electrons trapped in the inner magnetosphere was elevated for several weeks after the impacts. Magnetospheric physics is one of the many fields of scientific study that are now enjoying the benefit of new data provided by astronomical observations of this rare celestial event.
Jupiter's magnetosphere provides an important conceptual link between planetary magnetospheres, which are accessible to in situ observation, and remote astrophysical objects like pulsars, which are not. Our study of Jupiter's magnetosphere has been invigorated by the comet SL-9 impact and by recent advances in Earth-based observations and will be further invigorated by observations made with the Galileo spacecraft as it orbits Jupiter beginning in December 1995.
Acknowledgments: We thank Nick Schneider and John Trauger for providing the image shown in Figure 2. This work was supported in part by NSF grant ATM-9322360.
Source: Eos, Vol. 76, No.33, 8 Aug 1995, pp. 313-314
Physics of the Jovian Magnetosphere, edited by A. J. Dessler, Cambridge University Press, 1983.
"Plasma Motions in Planetary Magnetospheres," by T. W. Hill and A. J. Dessler, Science, vol. 252, pp. 410-415, 1991.
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