Jupiter and its moons

2.2 Jupiter's magnetic field and radiation zone

The latter part of Section 1.4 describes Jupiter's enormously strong magnetic field, and Section 1.3 mentions the associated radiation belt of magnetically-confined charged particles that are liable to damage the electronics of any spacecraft that lingers too long. Figure 2 shows views of the inner and most intense part of the radiation zone. Another version is shown in Figure 3.

Figure 2: Two views of Jupiter recorded about half a rotation (5 hours) apart by Cassini, a Saturn-bound mission that flew past Jupiter at the start of 2001. The planet is seen in visible light, but the colourful patches on either side indicate the intensity of radiation mapped by a detector of microwaves.
Figure 3: Jupiter's inner radiation belts. Details in radiation belts close to Jupiter are mapped from measurements that NASA's Cassini spacecraft made of radio emission from high-energy electrons moving at nearly the speed of light within the belts.
NASA/JPL

Figure 4 shows a greater extent of the radiation zone detected by a different method. A colour version of Figure 5 showing an aurora where the magnetic field channels charged particles into the atmosphere appears as Figure 6, and a more powerful and spectacular aurora recorded in the ultraviolet is shown in Figure 7.

Figure 4: The vast magnetosphere of (normally invisible) charged particles, whirling around Jupiter, here imaged by a new type of instrument aboard NASA's Cassini spacecraft
NASA/JPL/Johns Hopkins University Applied Physics Laboratory
Figure 5: The traditional view of Hadley cells in Jupiter's atmosphere. E means that there is flow to the east, and W to the west.
Figure 6: Visible evidence of Jupiter's internally-generated magnetic field: an aurora (glow from ionized atoms) seen by the spaceprobe Galileo on Jupiter's night-side. Lines of latitude and longitude have been superimposed (D.A. Rothery).
NASA/California Institute of Technology
Figure 7: Satellite footprints seen in aurora. This is a spectacular NASA Hubble Space Telescope close-up view of an electric-blue aurora that is eerily glowing one half billion miles away on the giant planet Jupiter. Auroras are curtains of light resulting from high-energy electrons racing along the planet's magnetic field into the upper atmosphere.
NASA and the Hubble Heritage Team (STScI/AURA). Acknowledgment: NASA/ESA, John Clarke (University of Michigan)

The strong radiation zone in Figure 2 is actually hoop-like in shape, and runs right round the planet. However, the figure makes it look like separate patches to either side of the globe because this is where our line of sight passes through the greatest extent of the zone. To visualize this, imagine sticking a knitting needle into a ring doughnut: a needle pointing towards the centre would only pass through a couple of centimetres before reaching the hole, whereas a needle parallel to this but off to the side would miss the hole and pass through a lot more cake before emerging again.

Question 1

Bearing in mind information given in Section 1.4, why do you think the location of the radiation belt relative to the planet appears to change between the two images in Figure 2?

Answer

The two images were obtained half a rotation apart, and the 'wobble' in the orientation of the radiation belt arises because the axis of the magnetic field is tilted relative to Jupiter's rotation axis (by nearly 10° according to Section 1.4.