Radio Emission from Jupiter

Abstract

Three reviews of radio emission from Jupiter have appeared recently (1, 2, 3) and at least one more is currently in press. The present review primarily covers work since 1961. It would be not too strong to suggest that explanations of Jupiter's emission now are in the second generation, with most observational phenomena fairly well established. The salient observed features are not widely known; this report will emphasize the author's evaluation of the relevant material. There is also presently a need to bring together the new material, for example, on conditions in the earth's mag­ netosphere, as they relate to Jupiter's radio emission. At the time of its discovery in 1954 and 1955, Jupiter's non thermal emission (then observed only in the decametric range) presented a deep puzzle, both because of its impulsive character (bursts of a fraction of one second up to a few seconds being commonly observed), and because of its strength, emanating from what had appeared to be a cold, albeit giant, planet. The explanations offered at that time centered around geophysical analogues, such as lightning in thunder storms or violent and frequent vol­ canic activity with consequent atmospheric shock waves. Inasmuch as non thermal radio astronomy was then in birth, the perspective of these times is difficult for us, only eight years later. Solar bursts had not yet been given more than a tentative explanation; the sun was considered to produce cosmic rays very rarely, once or twice a spot cycle during the greatest of flares; the Van Allen belts were unknown; synchrotron radiation from the sun had not been observed, nor its existence in the Crab nebula definitively established; and the most remarkable objects of all, the quasi-stellar radio sources, such as a 3C273, were yet to be discovered. Had this range of phenomena been known in 1955, without a doubt, the discoverers of Jupiter's decametric emission would have turned to the types of explanations currently proposed, which rely heavily on non thermal particles trapped in magnetic fields. There are at least four competitive theories of Jupiter's decametric emission, all of which rely broadly on the existence at Jupiter of a magnetic field that traps fast particles which are involved with both decimetric and decametric emission. One group of theories, by Carr (4), Ellis (5, 6), Ellis & McCulloch (7), and Field (8) explains decametric phenomena in terms of coupling effects between fast particles and decametric radio waves; these effects do not necessarily occur close to the planet. In a broad sense, these