Quadrupole and octupole parameters of Jupiter's main magnetic field

Abstract

Summary. The classical theory of multipoles is used to calculate the true quadrupole and octupole parameters for six different models of Jupiter's main magnetic field. These six magnetic-field models, which are based on measurements made by the Pioneer 10 and Pioneer 11 spacecraft, are specified in terms of the fifteen spherical harmonic coefficients required to define the Jovian dipole (3), quadrupole (5) and octupole (7). The set of five equations for the quadrupole parameters and the set of seven equations for the octupole parameters are each solved iteratively to give the corresponding true multipole moment and the directions of the associated multipole axes. It is found that the five quadrupole parameters are defined reasonably accurately by the Pioneer measurements, as are the three dipole parameters, but it is concluded that there are appreciable uncertainties in each of the seven octupole parameters. The true quadrupole and octupole magnetic moments are typically 14 and 5 per cent, respectively, of the dipole moment. These percentages are significantly different from the corresponding percentages for the pseudo-magnetic moments, namely 24 and 21 per cent, which are usually quoted in discussions of the higher multipoles of the Jovian magnetic field. Both the true and pseudo-multipole moments are larger for Jupiter than for the Earth. It is suggested finally that a proper multipole expansion of Jupiter's main magnetic field may have important applications in quantitative studies of several problems involving the Jovian magnetosphere, such as the decimetric and decametric radio emissions from Jupiter.