RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES3003, doi:10.2205/2005ES000175, 2005

Changes in the Magnetic Field Trend With the Characteristic Time Interval of ~100 years

[33]  As the magnetic moment declines, outbursts appear and grow in number, attaining their maximum values in the central region of the reversal, when the virtual geomagnetic poles (VGP) reside in the areas of intermediate and low latitudes. After they leave the medium- and low-latitude bands, the number of outbursts declines as the magnetic moment grows higher. The characteristic time of the outbursts lasts about 40-200 years, that is, almost coincides with the time of the accumulation of one to four sampling intervals of the rapidly accumulating rocks. In the central interval of the reversal, the outbursts sometimes follow one another, and the VGP movements grow highly disorderly to the extent that some researchers suggested their independence of the real field variations. The authors of this view believe that under the conditions of the low magnetic moment (0.1 and less of the present day M value) magnetization operated as a random process, and its direction was not controlled by the magnetic field which varied at that time in its direction extremely rapidly and randomly, or was absent at all [Vadkovskii et al., 1980]. This assumption of the absolute absence of the geomagnetic field contradicts the view of S. I. Braginskii, who supposed that the subsurface layer of the core (~20-30 km thick) was stratified and showed some special characteristics: the liquid core material differentiation resulted in the fact that the density of this layer was somewhat lower (at least by fractions of percent) than that of the major volume of the liquid core. The arising density gradient, which was estimated theoretically to be sufficient for changing the Reynolds magnetic number significantly and, hence, for creating the generation conditions, different from those existing in the major liquid core volume. S. I. Braginskii believes that single oscillations may be generated in this surface layer, distinguished by their characteristic torsion oscillation times and amplitudes typical of the "main spectrum'' oscillations. Also possible are periodical M declines to the level close to zero, yet the association of the vertical geomagnetic poles (VGP) movements with the low M value (close to zero) can be treated in a different way.

[34]  Oscillations with the typical periods of about 100 years and lower develop (according to S. I. Braginskii) in the surface layer of the liquid core. Since the magnetic field suppresses the movements of the conducting material, the intensity of the processes operating in the subsurface layer grows as the magnetic moment declines. The number of the outbursts grows slowly during the M decline, rather than beginning from some low M level, which is in better agreement with the second interpretation of the pattern observed. However, the combination of both factors is possible, namely, of the growing activity of the processes operating in the subsurface layer and of the low contribution of the magnetic field to the magnetization.

[35]  Be it as it may, it is precisely these random, occasionally continuous, processes (if they are real ones, rather than being the products of magnetization in the low magnetic field, or were produced by magnetization in some low magnetic field and by the development of natural remanent magnetization in sedimentary rocks) that often distinguish the inversion conditions from the other states of the geomagnetic field.


RJES

Citation: Gurarii, G. Z. (2005), Geomagnetic field reversals: Main results and basic problems, Russ. J. Earth Sci., 7, ES3003, doi:10.2205/2005ES000175.

Copyright 2005 by the Russian Journal of Earth Sciences

Powered by TeXWeb (Win32, v.2.0).