Abstract
Short-term constituents of the secular variation, at inter-decadal (20–30 years) and sub-centennial (60–90 years) time scales, present in observatory data and main field models, are also found in the radial field evolution at core surface. The paper is focused on the sub-centennial constituent in the gufm1 model. Time–Longitude (t–λ) plots, covering the 400 years time span of the model, at various latitudes between 70°N and 70°S, show a clear westward movement of the sub-centennial constituent field features in the 20°N–20°S latitude band. The sub-centennial constituent at latitudes larger than 50°N/S stands in fact for the fine structure of high-latitude flux lobes. Since 1900 this fine structure shows a westward displacement. Time–Latitude (t–φ) plots indicate also northward and southward components of the movement. The traveling speeds of the sub-centennial constituent field are derived, on one hand, empirically based on Time–Longitude and Time–Latitude plots, and on the other, mathematically by means of the Radon transform method. Important results of this paper are related to characterization of the evolution of the radial field at core surface at sub-centennial time scales, namely (1) evidencing two types of azimuthal flow, equatorial and high latitude ones, responsible for the observed westward drift of the surface field, and (2) quantitative information on meridional displacements of the core surface magnetic flux patches.
Highlights
The main geomagnetic field and its secular variation (SV) are, together with geodynamo modeling, a source of information on possible movements of the conductive fluid of the outer core that lead to the generation of the field
Geographical distribution of the sub‐centennial constituent at the top of the core Maps at various times in the 320 years interval (1630– 1950) of the sub-centennial constituent show positive and negative magnetic flux patches that migrate in space and time
Using a filtering approach that successively eliminates oscillatory behavior at certain timescales, present in the gufm1 radial field at core surface, we showed that a sub-centennial oscillation is responsible for the fine structure of the time evolution of the geomagnetic field at the core-mantle boundary
Summary
The main geomagnetic field and its secular variation (SV) are, together with geodynamo modeling, a source of information on possible movements of the conductive fluid of the outer core that lead to the generation of the field. We would like to stress that our t–λ plots include two types of information: (a) information on field flux patches direction and speed of movement which is, in the equatorial band, similar to that previously found by Finlay and Jackson (2003) for oscillations in a time window of 400 years, and (b) information on intensity variations of both axisymmetric and nonaxisymmetric parts of the field, as the sub-centennial constituent is present in all terms of the time-dependent spherical harmonic expansion that describes the geographical distribution and the time evolution of the core field.
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