In 1955, when radio astronomy was still a comparatively new science, astronomers near Washington DC were looking for previously undiscovered radio sources of small apparent size. Their telescope picked up strong waves at a frequency of 22.2MHz, corresponding to a wavelength of 13.5metres. At first they attributed these radio waves to terrestrial sources such as faulty car-ignition systems. The interference persisted and continued observations showed that the source moved relative to the stars and that it was in fact Jupiter. Planets were not then considered likely radio sources and so it was several months before this identification was made – although Jupiter was shining brightly overhead for all to see. The nature of the observed radio waves indicated the existence of a magnetic field and energetic particles around Jupiter. Both of these were first directly observed by the Pioneer 10 spacecraft in late 1973.

A description of the RADIO EMISSION FROM JUPITER can be divided conveniently into three parts. At wavelengths shorter than 7cm the emission is mainly thermal in origin and comes from the body of the planet. At longer wavelengths, the emission is primarily non-thermal and has two clearly distinguishable components. One is a centimeter and decimetre component, with wavelengths below about one meter, and the other is a decametre component, which is important at wavelengths longer than 7.5 meters.

The DECAMETRE RADIATION component has been detected at frequencies between 450 kHz and 39.5 MHz, i.e. wavelengths between 670 and 7.5 meters . At the longer end of this wavelength range the radio waves cannot penetrate the Earth’s own ionosphere and the observations have been made from Earth-orbiting satellites. Unlike the steady emission below one meter, the decametre radiation is emitted sporadically in short intense bursts of radio noise. The emission is strongest around 10MHz (30m). Many bursts occur during a period, usually between several minutes and several hours long, to form a JOVIAN NOISE STORM. Between the storms are quiescent periods which may last for hours, days or weeks. Most of the individual bursts last between 0.5 and 5 seconds and are known as L BURSTS. Shorter (s BURSTS) and longer bursts also occur. Each burst contains just a narrow band of frequencies somewhere within the total observed range for all bursts. The decametre emissions are highly polarized; it is likely that they are polarized at the point of origin but that the magnetic field of Jupiter substantially modifies the polarization of the radiation on its way to the Earth.

A characteristic feature of these bursts is that they occur preferentially when the longitude (System II) of the central meridian has certain values, which implies that the emission is coming from certain preferred longitude zones. These are not identical at all frequencies but at 18MHz three zones A, B and C have been identified. The constancy of longitude in System II is not exact and a radio rotation period of 9hr 55min 29.37 sec was adopted in 1962 as a best fit to the observations. This is the System III referred to above. As mentioned there, a better value of 9hr 55mm 29.75sec has been obtained from more recent observations.

The decametre emissions are also affected by the satellites lo and, to a lesser extent, Europa. Bursts from zones A and B are most likely to occur when lo is in certain positions in its orbit . Not all bursts, however, occur when lo is near these positions. The explanation for this effect is unknown but it seems likely to be connected in some way with the ionosphere of lo which was detected by Pioneer 10. Also unexplained is the origin of the decametre radiation itself. Although it is known that individual bursts probably come from regions less than 400km across, the exact positions are not known. This lack of important information is a main reason for the origin remaining unexplained.
The shorter CENTIMETRE and DECIMETRE wavelength RADIO RADIATION from Jupiter is quite different from the decametre emission. Between about 5 and 300cm, the radiated power is constant and comes from an area much larger than the planetary disc. The emitting region sways as the planet rotates. The radiation is linearly polarized and the direction of polarization rocks back and forth 10° each side of its mean direction. From observations such as these a rotation period can be obtained; it is the same as that obtained from the longer, decametre emission. These Earth-based observations strongly suggest that, like the Earth, Jupiter has a magnetic field and belts of energetic particles and that the centimeter and decimeter radio emission is synchrotron radiation. The existence of the magnetic field and the energetic particles have now been confirmed by spacecraft measurements.

Jupiter has an intrinsic MAGNETIC FIELD which is described reasonably accurately by a magnetic dipole. The dipole is tilted 10°.77 to the rotation axis towards (in December 1974) System III longitude 230°.9. The dipole is offset from the centre of Jupiter by a distance of 0.101 times the planet’s radius in the direction of latitude 5°. 12 north and System III longitude 185°.7. The dipole currently points in the opposite direction to that of the Earth so that a terrestrial compass would point towards Jupiter’s south pole. The maximum field at Jupiter’s visible surface is about 20 times greater than the corresponding value for the Earth. Because of the tilt of the magnetic field relative to the spin axis, any phenomena connected with the field will show a wobble as Jupiter rotates. This applies particularly to the radio emission and so the rotation period measured from radio observations is in fact the rotation period of the magnetic field. Since the central parts of Jupiter are liquid, the magnetic field is most probably generated by the same dynamo effect that is thought to be responsible for the Earth’s field.

Jupiter’s MAGNETOSPHERE, as the region where Jupiter’s magnetic field dominates over that of the Sun is called, has several basic parts . In the inner magnetosphere, which ex¬tends about 20 Jupiter radii from the planet, the field of the dipole predominates. The rotation of the field closely follows that of the dipole. The observed field is generally relatively smooth except that significant perturbations caused by the satellites Europa and Ganymede, whose orbits are in this part of the magnetosphere, have been observed. In the inner region the magnetosphere behaves in a similar way to that of the Earth.

Further out from Jupiter, the field is much more complicated. In the middle magnetosphere, between 20 and 50 Jupiter radii from the planet, there is a thin, disc-like region in which the magnetic field is nearly constant at 10-8 tesla. The field traps a low-energy plasma. Electric currents flow in this, forming what is called a current sheet, and are responsible for the field. Because of the tilt of Jupiter’s magnetic axis, this sheet is warped so that it is above the equator on one side of the planet and below it on the other. Like the inner magnetic field this warped sheet rotates with the planet. Away from this disc-like region, the field in the middle magnetosphere is weaker and more like that of a dipole. In the outer magnetosphere the field is very irregular and, as with the Earth, it extends away from the Sun in a comet-like tail. The distance-of the magnetosphere boundary from Jupiter towards the Sun varies considerably in short times between extreme values around 50 and 100 Jupiter radii.

Jupiter’s magnetic field traps energetic particles, mainly electrons and protons, in the same way as the Earth’s does although Jupiter’s radiation belts appear to be 10 000 times more intense than the Earth’s Van Alien belts. In the middle magneto-sphere the particles are concentrated into the same disc-like region as the magnetic field. The particles in this outer radiation belt are probably not completely trapped so therefore they continually leak away. At the same time other particles are captured from the interplanetary medium. There is also an inner radiation belt of particles trapped in the inner magnetosphere, but unlike those in the outer belt the particles here are firmly trapped. As well as the energetic particles, Jupiter’s magnetosphere contains a plasma of low-energy particles which is not confined to the current sheet of the middle magnetosphere.

As mentioned above, astronomers deduced that the centimeter and decimeter radio emission from Jupiter was due to synchrotron radiation produced by energetic charged particles moving in a magnetic field. These particles can now be identified with those detected in the inner magnetosphere by the Pioneer spacecraft. The strength of the synchrotron radiation depends on both the energy of the charged particles and the strength of the magnetic field. The Pioneer observations confirmed that the particle energies and magnetic field strength have the correct values to produce the observed radio emission.

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