The decrease of the geomagnetic dipole field as part of the general secular variation.

Abstract

The ongoing decrease of the earth's dipole moment has repeatedly been interpreted as a decay of the whole geomagnetic field. However, the dipole part of the secular variation loses its predominant role, if the field is reduced to the depth of the source layer, some 100km below the core/mantle boundary, as inferred from the spatial energy density spectrum of the field. It is shown that the mean magnetic energy density at that depth is already balanced if only two more spectrum terms, i.e., the quadrupole and the octupole constituents, are included. This result strongly supports the concept of the presently observed secular variation to be caused primarily by structural changes of the current system in the source layer, associated with an exchange of energy between spherical harmonics of different degrees, without involving a growth or decay of the field as a whole.