11.4 Jupiters Magnetosphere
Direct spacecraft measurements show Jupiters magnetosphere to be almost 30 million km across, roughly a million times more voluminous than Earths magnetosphere, and far larger than the entire Sun. As with Earths, the size and shape of Jupiters magnetosphere is determined by the interaction between the planets magnetic field and the solar wind. Jupiters magnetosphere has a long tail extending away from the Sun at least as far as Saturns orbit (over 4 A.U. farther out from the Sun), as sketched in Figure 11.11. However, on the sunward side, the magnetopausethe boundary of Jupiters magnetic influence on the solar windlies only 3 million km from the planet. Near the planets surface, the planets magnetic field channels particles from the magnetosphere into the upper atmosphere, forming aurorae vastly larger and more energetic than those observed on Earth (Figure 11.12). (Sec. 7.5)
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, Jupiters rapid rotation has forced most of the charged particles into a flat current sheet, lying on the planets magnetic equator. The portion of the magnetosphere close to Jupiter is sketched in Figure 11.13. Notice that the planets magnetic axis is not exactly aligned with its rotation axis but is inclined to it at an angle of approximately 10°. The planets magnetic field happens to be oriented opposite Earths, with field lines running from north to south, rather than south to north as in the case of our own planet (see Figure 7.21).
Both ground- and space-based observations of the radiation emitted from Jupiters magnetosphere imply that the intrinsic strength of the planets magnetic field is nearly 20,000 times greater than Earths. The existence of such a strong field further supports our theoretical model of Jupiters internal structure. The conducting liquid interior that is thought to make up most of the planet should combine with Jupiters rapid rotation to produce a large dynamo effect and a strong magnetic field, just as observed. (Sec. 7.4)
Why is Jupiters magnetosphere so much larger than Earths?