On paleomagnetic data and dynamo theory.

Sufficient paleomagnetic data of wide distribution now exist for their use in the determination of terms of higher degree than dipole in the past geomagnetic field .For times older than 5Myr it is necessary to carry out global reconstructions based upon information from sea-floor spreading.Best fits of second and third degree zonal terms (quadrupole and octupole), normalised by the dipole term, may then be obtained for spherical harmonic descriptions of the past geomagnetic field.Such paleomagnetic results are examined for their significance for dynamo theory, using the analysis developed in terms of spherical harmonics by BULLARD and GELLMAN (1954), and applied to mean field electrodynamics by RoBERTS and STIx (1972).The paleomagnetic results provide data for ratios of the values at the coremantle boundary of the radial functions which describe the tonal dipole, quadrupole and octupole geomagnetic components.The variations with time of these ratios over the last 200Myr indicate an evolution of the geodynamo, and show that the octupole geomagnetic component may be linked to the dipole geomagnetic component in a more steady manner than is the quadrupole geomagnetic component.One possible interpretation of such an observation is that it indicates different patterns of behaviour between quadrupole and dipole-octupole fluid flow components in the earth's core.