Broca's Brain - Carl Sagan [129]
Subsequently Jupiter was found to be a bright source at decimeter wavelengths as well. But the spectrum was very peculiar. At a wavelength of a few centimeters, very low temperatures of around 140°K were found—temperatures comparable to those uncovered for Jupiter at infrared wavelengths. But at decimeter wavelengths—up to one meter—the brightness temperature increased very rapidly with wavelength, approaching 100,000°K. This was too high a temperature for thermal emission—the radio radiation that all objects put out, simply because they are at a temperature above absolute zero.
Frank Drake, then of the National Radio Astronomy Observatory, proposed in 1959 that this spectrum implied that Jupiter was a source of synchrotron emission—the radiation that charged particles emit in their direction of motion when traveling close to the speed of light. On Earth, synchrotrons are convenient devices used in nuclear physics for accelerating electrons and protons to such high velocities, and it is in synchrotrons that such emission was first generally studied. Synchrotron emission is polarized, and the fact that the decimeter radiation from Jupiter is also polarized was an additional point in favor of Drake’s hypothesis. Drake suggested that Jupiter was surrounded by a vast belt of relativistic charged particles similar to the Van Allen radiation belt around the Earth, which had then just been discovered. If so, the decimeter emitting region should be much larger than the optical size of Jupiter. But conventional radio telescopes have inadequate angular resolution to make out any spatial detail whatever at the range of Jupiter. A radio interferometer can achieve such resolution, however. In the spring of 1960, very soon after the suggestion was made, V. Radhakrishnan and his colleagues at the California Institute of Technology employed an interferometer composed of two 90-foot-diameter antennas mounted on railroad tracks and separable by almost a third of a mile. They found that the region of decimeter emission around Jupiter was considerably larger than the ordinary optical disc of Jupiter, confirming Drake’s proposal.
Subsequent higher-resolution radio interferometry has shown Jupiter to be flanked by two symmetric “ears” of radio-wave emission with the same general configuration as the Van Allen radiation belts of the Earth. The general picture has evolved that on both planets electrons and protons from the solar wind are trapped and accelerated by the planetary magnetic dipole field and are constrained to spiral along the planet’s lines of magnetic force, bouncing from one magnetic pole to the other. The radio-emitting region around Jupiter is identified with its magnetosphere. The stronger the magnetic field, the farther out from the planet the boundary of the magnetic field will be. In addition, matching the observed radio spectrum from synchrotron emission theory specifies a magnetic field strength. The field strength could not be specified to very great precision but most estimates from radio astronomy in the late 1960s and early 1970s were in the range of 5 to 30 gauss, some ten to sixty times the surface magnetic field of the Earth at the equator.
Radhakrishnan and colleagues also found that the polarization of the decimeter waves from Jupiter varied regularly as the planet rotated, as if the Jovian radiation belts were wobbling with respect to the line of sight. They proposed that this was due to a 9-degree tilt