Nature 389, 371 - 374 (25 September 1997); doi:10.1038/38712
WEIJIA KUANG1 AND JEREMY BLOXHAM1
Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA
http://www.nature.com/nature/journal/v389/n6649/full/389371a0_fs.html
The mechanism by which the Earth and other planets maintain their magnetic fields against ohmic decay is among the longest standing problems in planetary science. Although it is widely acknowledged that these fields are maintained by dynamo action, the mechanism by which the dynamo operates is in large part not understood. Numerical simulations of the dynamo process in the Earth's core1-4 have produced magnetic fields that resemble the Earth's field, but it is unclear whether these models accurately represent the extremely low values of viscosity believed to be appropriate to the core. Here we describe the results of a numerical investigation of the dynamo process that adopts an alternative approach5 to this problem in which, through the judicious choice of boundary conditions, the effects of viscosity are rendered unimportant. We thereby obtain a solution that at leading order operates in an Earth-like dynamical regime. The morphology and evolution of the magnetic field and the fluid flow at the core-mantle boundary are similar to those of the Earth, and the field within the core is qualitatively similar to that proposed on theoretical grounds6.
Figure 1 A segment of a magnetic field line illustrating the dynamo process. The outer spherical surface with the latitude-longitude grid is the core-mantle boundary and the inner red surface is the inner-core boundary; the rotation axis is the straight line passing through the inner core. The field line is colour-coded so that it is yellow where it has a positive radial component and blue where it has a negative radial component. The field-generating mechanism can be visualized as follows. Starting at the top on the large loop of field protruding from the core, and tracing down and to the right, the field line enters the core, at the rear of the core in this view. Then, the field line is stretched around the rotation axis by the differential rotation (the mechanism by which the strong toroidal field is generated). Then, the effects of the poloidal part of the flow on the field can be seen, as the field line is twisted and spirals downwards almost parallel to the rotation axis. The effect of this helical distortion of the field line is apparent from the alternation between blue and orange along the field line as it is twisted. Next, the field line is again stretched by the differential rotation, before re-emerging from the core on the right-hand side. The field line then loops outside the core before re-entering the core (near the top right). On re-entering the core, this cycle repeats. This classical picture of generation of toroidal field by differential rotation and generation of poloidal field by helical motions was first postulated more than 40 years ago6. Starting again from the original starting point, but following the large loop in the other direction, we can see the field line penetrate the inner core.
Science 13 December 1996:
Vol. 274. no. 5294, pp. 1887 - 1891
DOI: 10.1126/science.274.5294.1887
http://www.nature.com/nature/journal/v389/n6649/full/389371a0_fs.html
Gary A. Glatzmaier * and Paul H. Roberts
Three-dimensional numerical simulations of the geodynamo suggest that a superrotation of Earth's solid inner core relative to the mantle is maintained by magnetic coupling between the inner core and an eastward thermal wind in the fluid outer core. This mechanism, which is analogous to a synchronous motor, also plays a fundamental role in the generation of Earth's magnetic field.
G. A. Glatzmaier, Institute of Geophysics and Planetary Physics, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. E-mail: gag@lanl.gov
P. H. Roberts, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095, USA. E-mail: roberts@math.ucla.edu
115-120 ky
1,8 My (Upper Olduvai)
6.5 My
11 My
http://www.nature.com/nature/journal/v394/n6696/abs/394878a0_fs.html
Nature 394, 878 - 881 (27 August 1998); doi:10.1038/29746
M. T. JUÁREZ*, L. TAUXE†, J. S. GEE† & T. PICK‡
* Fort Hoofddijk Paleomagnetic Laboratory, Budapestlaan 17, 3584 CD Utrecht, The Netherlands
† Scripps Institution of Oceanography, La Jolla, California 92093-0220, USA
‡ European Topic Center on Catalogue of Data Sources, Archivstrasse 2, D-30169 Hannover, Germany
Correspondence and requests for materials should be addressed to L.T. (e-mail: ltauxe@ucsd.edu).
Present field twice that of the Mezozoic dinosauri era (160 My)
To augment the palaeointensity database, we focused on submarine basaltic glass (SBG) obtained from 20 sites sampled by the Deep Sea Drilling Project (DSDP). In addition to the DSDP glasses, we also include results from SBG obtained from pillow lavas in the Troodos ophiolite on Cyprus. SBG has been shown to contain predominantly single-domain magnetite as the carrier of the remanent magnetization7,8, based on a detailed rock-magnetic analysis.
Maximum blocking temperatures of the NRM for most of the samples are generally between 475 and 550 °C, i