The size of Jupiter's electrically conducting fluid core

Abstract

WHEN the magnetic field of a planet is due to hydromagnetic dynamo action in an electrically conducting fluid core surrounded by a poorly conducting mantle, Hide's¹ method can, in principle, be used to determine the radius r_c of the core from determinations of secular changes in the magnetic field B in the accessible region above the surface of the planet, mean radius rs r_s (⩾r_c). It will be possible to apply this method to Jupiter (and other planets) when magnetic measurements of sufficient accuracy and detail become available. We describe here a preliminary study in which we have analysed the magnetic measurements made in December 1973 and December 1974 when the Pioneer 10 and Pioneer 11 fly-by space-probes encountered Jupiter². We expected that over such a short time interval any true secular changes would be masked by errors and the corresponding estimate of r_c/r_s highly implausible or even physically impossible, but this turns out not to be the case. Taken at their face value the apparent secular changes in the dipole and quadrupole components of Jupiter's magnetic field imply that r_c/r_s is close to 0.7. Somewhat higher values of r_c/r_s are found when contributions from the octupole component are also included.