Inner core anisotropy, anomalies in the time-averaged paleomagnetic field, and polarity transition paths

Abstract

The diffusion of the dynamo-generated magnetic field into the electrically conducting inner core of the Earth may provide an explanation for several problematic aspects of long-term geomagnetic field behavior. We present a simple model which illustrates how an induced magnetization in the inner core which changes on diffusive timescales can provide a biasing field which could produce the observed anomalies in the time-averaged field and polarity reversals. The Earth's inner core exhibits an anisotropy in seismic velocities which can be explained by a preferred orientation of a polycrystalline aggregate of hexagonal close-packed (hcp) iron, an elastically anisotropic phase. Room temperature analogs of hcp iron also exhibit a strong anisotropy of magnetic susceptibility, ranging from 15 to 40% anisotropy. At inner core conditions the magnetic susceptibility of hcp iron is estimated to be between 10m4 and 10e3 SI. We speculate here that the anisotropy in magnetic susceptibility in the inner core could produce the observed anomalies in the time-averaged paleomagnetic field, polarity asymmetry, and recurring transitional virtual geomagnetic pole (VGP) positions.

Electronic topological transition and noncollinear magnetism in compressed hcp Co

“…Such compressions roughly correspond to the state of the matter close to the Earth's core, so that the processes occurring under the deep mantle can be reconstructed in present day laboratories [2,3]. Particularly, the investigation of magnetic systems is of interest, because it can give fundamental insights on the origins of the geomagnetism [4].…”

The time-averaged geomagnetic field: global and regional biases for 0-5 Ma

“…1991;Runcorn 1992). More recently it has been argued that VGPs may track in different bands for each reversal, indicating a possibility of shorter time-constant controls exerted by the inner core on field structure (Clement & Stixrude 1995). Hemispheric differences in the TAF, if they exist, should be manifest in palaeomagnetic data from stable polarity intervals, which are generally more reliable, as well as more numerous, than those from times when the field is reversing.…”

Magnetic Properties of Hexagonal Closed-Packed Iron Deduced from Direct Observations in a Diamond Anvil Cell

“…Recent numerical models suggest that a finitely conducting solid inner core stabilizes the geodynamo (1,2). Deviations of the mean field from that of a geocentric dipole, such as the far-sided effect (3), could be explained if the inner core is composed of an aggregate of preferentially oriented hcp iron crystals (4). Such an aggregate could also explain why se~smic waves traveling parallel to the rotational axis annear 1 to 4% faster than those traversing equatorial paths (5).…”