Origin of Jupiter's Magnetic Field

Abstract

Plausible theories of non-thermal radio-noise from Jupiter on decimetre and decametre wavelengths invoke a strong and nearly dipolar Jovian magnetic field and an associated system of van Allen-type radiation belts of electrically-charged particles extending beyond and interacting with the first Galilean satellite 10. Data from the recent Pioneer I0 fly-by provide direct evidence for the radiation belts and magnetic field and confirm the values for the strength, position and orientation of the equivalent Jovian magnetic dipole inferred previously by radio astronomers (Smith et al. 1974; Stannard 1975). The apparent absence of a non-thermal component in Saturn’s radio-emission implies that radiation belts cannot form around that planet, possibly because Saturn is non-magnetic, or, more likely, because charged particles in the vicinity of Saturn are rapidly removed through interactions with the rocky fragments that make up Saturn’s rings. Considerations of possible sources of Jupiter’s magnetic field-which theories of Jupiter’s internal constitution and energy sources must now take properly into account -indicate that the most likely mechanism is a magnetohydrodynamic dynamo associated with thermally-driven fluid motions in the electrically-conducting parts of Jupiter’s interior (Hide 1965; see also Runcorn 1968; Smoluchowski 1971 ; Stevenson 1974). It has not yet proved possible in dynamo theory to establish a precise relationship between the strength of the observed magnetic field and other parameters (density, planetary radius, electrical conductivity, rotation rate, etc.), but it proves instructive to compare the observed strength of Jupiter’s magnetic field with approximate theoretical relationships based on general considerations of the equations of magnetohydrodynamics of rotating fluids (Hide 1974) or on more specific dynamo models such as Parker’s (Levy 1972; Roberts 1975).