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By Root 1274 0

between the axis of rotation and the magnetic axis of the planet—not very different from the displacement between the north geographic and the north magnetic poles of Earth. Subsequent studies of the decimeter and decameter emission by James Warwick of the University of Colorado and others suggested that the magnetic axis of Jupiter is displaced a small fraction of a Jupiter radius from the axis of rotation, quite different from the terrestrial case, where both axes intersect at the center of the Earth. It was also concluded that the south magnetic pole of Jupiter was in the northern hemisphere; that is, that a north-seeking compass on Jupiter would point south. There is nothing very bizarre about this suggestion. The Earth’s magnetic field has flipped its direction many times during its history, and it is only by definition that the north magnetic pole is in the northern hemisphere of the Earth at the present time. From the intensity of the decimeter and decameter emission, astronomers also calculated what the energies and fluxes of electrons and protons in the Jovian magnetosphere might be.

This is a very rich array of conclusions. But all of it is remarkably inferential. The whole elaborate superstructure was put to a critical test on December 3, 1973, when the Pioneer 10 spacecraft flew through the Jovian magnetosphere. There were magnetometers aboard, which measured the strength and direction of the magnetic field at various positions in the magneto-sphere; and there was a variety of charged-particle detectors, which measured energies and fluxes of the trapped electrons and protons. It is a stunning fact that virtually every one of the radio astronomical inferences was roughly confirmed by Pioneer 10 and its successor spacecraft, Pioneer 11. The surface equatorial magnetic field on Jupiter is about 6 gauss and larger at the poles. The inclination of the magnetic to the rotational axis is about 10 degrees. The magnetic axis can be described as apparently displaced about one quarter of a Jovian radius from the center of the planet. Farther out than three Jupiter radii, the magnetic field is approximately that of a dipole; closer in, it is much more complex than had been estimated.

The flux of charged particles received by Pioneer 10 along its trajectory through the magnetosphere was considerably larger than had been anticipated—but not so large as to inactivate the spacecraft. The survival of Pioneer 10 and 11 through the Jovian magnetosphere was more the result of good luck and good engineering than of the accuracy of pre-Pioneer magnetospheric theories.

In general, the synchrotron theory of the decimeter emission from Jupiter is confirmed. All those radio astronomers turn out to have known what they were doing. We can now believe, with much greater confidence than heretofore, deductions made from synchrotron physics and applied to other, more distant and less accessible comic objects, such as pulsars, quasars or supernova remnants. In fact, the theories can now be recalibrated and their accuracy improved. Theoretical radio astronomy has for the first time been put to a critical experimental test—and it has passed with flying colors. Of the many major findings by Pioneer 10 and 11, I think this is its greatest triumph: it has confirmed our understanding of an important branch of cosmic physics.

There is much about the Jovian magnetosphere and radio emissions that we still do not understand. The details of the decameter emissions are still deeply mysterious. Why are there localized sources of decameter emission on Jupiter probably less than 100 kilometers in size? What are these emission sources? Why do the decameter emission regions rotate about the planet with a very high time precision—better than seven significant figures—but different from the rotation periods of visible features in the Jovian clouds? Why do the decameter bursts have a very intricate (submillisecond) fine structure? Why are the decameter sources beamed—that is, not emitting in all directions equally? Why are the decameter sources intermittent