The Magnetosphere

Giant Jupiter has an enormous realm—from the size of its satellite system to its tremendous aurorae and superbolts of lightning, to the huge planet-sized cloud features that surround its atmosphere. The most gargantuan Jovian feature is its magnetosphere, which envelopes the satellites and constantly changes in size, pumping in and out at the whim of the solar wind. The Pioneer and Voyager spacecraft provided four cuts through this dynamic region, showing that its borders in the upwind solar direction lie between 50 R_J and 100 R_J from Jupiter. Downwind, away from the Sun, the magnetosphere extends much farther; some scientists postulate that a magnetotail may reach as far as the orbit of Saturn.

Charged particles in the magnetosphere are subject to powerful forces. Tightly embedded in Jupiter, the magnetic field spins with a ten-hour period as the planet rotates. The particles are caught in the spinning field and accelerated to high speeds. The result is a co-rotating plasma in the magnetic equator of Jupiter, extending outward to at least 20 R_J. Beyond this distance, the flow breaks up and the magnetosphere is more unstable. Within the co-rotation region, the spinning plasma sets up a powerful electric current girdling the planet.

Charged particles can be accelerated in the magnetosphere to high energies, corresponding to speeds tens of thousands of kilometers per second. Some of these particle streams escape from the inner parts of the magnetosphere and can penetrate the magnetopause and be ejected from the Jovian system. On Voyager 1, the low energy charged particle instrument began detecting these streams of “hot” plasma on 22 January, when Voyager 1 was still 600 R_J (almost 50 million kilometers) from the planet. Voyager 2 first detected Jovian particles at an even greater distance, 800 R_J. Hydrogen and helium ions (protons and alpha particles) dominate the magnetosphere at great distances from Jupiter, but increasing amounts of sulfur and oxygen appeared as the spacecraft crossed the magnetopause. The heavier ions presumably originate from Io.

In the inner magnetosphere, the Galilean satellites have a powerful influence on the populations of fast-moving particles. During the Voyager 1 encounter, the primary effect was observed at Io, where the satellite apparently sweeps up energetic electrons. In the million-volt energy range, these particles are depleted near Io, with peaks observed both inside and outside the satellite’s orbit. Voyager 2 passed close to Ganymede, and here also major effects were seen, with the satellite apparently absorbing electrons. In the wake created by the motion of the co-rotating plasma past Ganymede, the particle populations showed large and complex variations.

The magnetosphere of Jupiter can be “seen” from Earth by its emissions at radio wavelengths. The recent development of imaging radio telescopes in Great Britain, The Netherlands, and the United States allows frequent mapping of the large-scale features of the innermost magnetosphere, inside the orbit of Io. These six images showing one rotation of Jupiter were obtained near the time of the Voyager 1 flyby by Imke de Pater at the Westerbork Radio Observatory near Leiden. The brightest emission is shown by dark red or black. The size of the planet is indicated by a dotted white circle. The tilt of the magnetosphere relative to the rotation axis of the planet can be seen by the wobble of the magnetosphere as Jupiter rotates.

Just inside the Jovian magnetosphere is the “hot spot” of the solar system: a 300-400 million degree plasma detected by Voyager 1 while it was still about 5 million kilometers from Jupiter. T. P. Armstrong commented, “Even the interior of the Sun is estimated to be less than 20 million degrees.” S. M. Krimigis added that the temperature of this plasma is “the highest yet measured anywhere in the solar system.” Fortunately for Voyager, this region of incredibly hot plasma is also one of the solar system’s best vacuums. The spacecraft was in little danger because the bow shock protects this region from the solar wind, and most of the particles in Jupiter’s magnetosphere are held in much closer to the planet.

Each Voyager passed through the boundaries of the magnetosphere—the bow shock (BS) and the magnetopause (MP)—on both the inbound and the outbound legs of its passage through the Jovian system. In this diagram, the heavy solid line represents the spacecraft trajectory, as seen looking down from the north. Also shown are the positions of the bow shock in March and of the magnetosphere in both March and July.

Several regions of plasma (charged particles) make up the Jovian magnetosphere. These sketches, based on Voyager data, show the magnetosphere as viewed from above (a) and as seen from the Jovian equatorial plane (b). Most of the plasma co-rotates with the planet and is confined near the magnetic equator, where it forms a broad plasma sheet about 100 R_J across.

The very hot plasma in the outer magnetosphere discovered by Voyager is thought to play an important role in establishing the size of the Jovian magnetosphere. Although the density is low, only about one charged particle per hundred cubic centimeters, this plasma actually carries a great deal of energy because of the high speed of the particles. It is this plasma pressure, rather than the magnetic field pressure, that appears to hold off the pressure of the solar wind. However, the balance between hot plasma inside the magnetopause and the solar wind outside is not very stable. The Voyager experimenters suggest that a small change in solar wind pressure can cause the boundary to become suddenly unstable. A large quantity of the hot plasma can then be lost, producing the bursts seen at large distances and permitting a sudden collapse of the outer magnetosphere. Continued injection of hot plasma from within would then reinflate the magnetosphere, which would expand like a balloon until another instability developed. Processes of this sort may be the cause of the rapidly varying magnetospheric boundaries observed by both Voyager spacecraft.

The rings of Jupiter are best seen when looking nearly in the direction of the Sun, since the small particles that comprise them are good forward scatterers of sunlight. This mosaic is of Voyager 2 images (two wide angle and four narrow angle) obtained from a perspective behind the planet and inside the shadow of Jupiter. The spacecraft was 2 degrees below the equator of Jupiter and 1.5 million kilometers from the rings. The shadow of the planet can be seen to obscure the near segment of the ring near the edge of the planet. The brightest region of the ring is about 1.8 R_J from the center of Jupiter. [260-678B]