For decades, ground-based radio telescopes monitored radiation leaking from Jupiter's magnetosphere, but only when the Pioneer and Voyager spacecraft reconnoitered the planet in the mid-1970s did astronomers realize the full extent of the planet's magnetic field. Jupiter, it turns out, is surrounded by a vast sea of energetic charged particles, mostly electrons and protons, somewhat similar to Earth's Van Allen belts but much, much larger. The radio radiation detected on Earth is emitted when these particles are accelerated to very high speeds—close to the speed of light—by Jupiter's powerful magnetic field. This radiation is several thousand times more intense than that produced by Earth's magnetic field and represents a potentially serious hazard for manned and unmanned space vehicles alike. Sensitive electronic equipment (not to mention even more sensitive human bodies) would require special protective shielding to operate for long in this hostile environment.

Direct spacecraft measurements show Jupiter's magnetosphere to be almost 30 million km across, roughly a million times more voluminous than Earth's magnetosphere, and far larger than the entire Sun. As with Earth's, the size and shape of Jupiter's magnetosphere is determined by the interaction between the planet's magnetic field and the solar wind. Jupiter's magnetosphere has a long tail extending away from the Sun at least as far as Saturn's orbit (over 4 A.U. farther out from the Sun), as sketched in Figure 11.11. However, on the sunward side, the magnetopause—the boundary of Jupiter's magnetic influence on the solar wind—lies only 3 million km from the planet.

Figure 11.11 The Pioneer 10 spacecraft did not detect any solar particles while moving behind Jupiter. Accordingly, as sketched here, Jupiter's magnetosphere apparently extends beyond the orbit of Saturn.

The outer magnetosphere appears to be quite unstable, sometimes deflating in response to "gusts" in the solar wind, then reexpanding as the wind subsides. In the inner magnetosphere, Jupiter's rapid rotation has forced most of the charged particles into a flat current sheet, lying on the planet's magnetic equator. The portion of the magnetosphere close to Jupiter is sketched in Figure 11.12. Notice that the planet's magnetic axis is not exactly aligned with its rotation axis but is inclined to it at an angle of approximately 10°. The planet's magnetic field happens to be oriented opposite Earth's (see also Figure 11.18).

Figure 11.12 Jupiter's inner magnetosphere is characterized by a flat current sheet, consisting of charged particles squeezed into the magnetic equatorial plane by the planet's rapid rotation. The plasma torus is a ring of charged particles associated with the moon Io; it is discussed in Section 11.5.

Both ground- and space-based observations of the radiation emitted from Jupiter's magnetosphere imply that the intrinsic strength of the planet's magnetic field is nearly 20,000 times greater than Earth's. The existence of such a strong field further supports our theoretical model of the interior; the conducting liquid interior that is thought to make up most of the planet should combine with Jupiter's rapid rotation to produce a large dynamo effect and a strong magnetic field, just as observed. (Sec. 7.4)