| Popis: |
Paleomagnetic observations have provided much information about the nature of geomagnetic dipole reversals and three-dimensional (3-D) magnetohydrodynamic (MHD) computer models are beginning to provide insight to the reversal mechanism. Reversals typically take 2000–12 000 years to occur – one to two orders of magnitude shorter than the dipole-dominated, stable polarity epochs between reversals. However, the durations of both the polarity transitions and epochs vary greatly. Average reversal frequency appears to depend on the degree of equatorial symmetry of the nonaxial dipole field (i.e., the total field minus the axial dipole term). Virtually, all records document lowered intensity during reversals, but they differ in complexity of transitional direction change. At least some reversals have highly complex transition paths. In addition, stable polarity epochs are often punctuated by large directional excursions that occur more frequently than reversals and may be regarded as aborted reversals. A statistical bias of transition poles for preferred geographical regions that correlate with lower-mantle tomography has been claimed, and such bias has been produced in dynamo simulations employing laterally varying heat flux at the core–mantle boundary. No external trigger is necessary for reversals, which occur as a natural result of the very nonlinear dynamo process; that is, the flow, field, and thermodynamic variables of the Earth’s outer fluid core all depend on time and space and on each other in a chaotic way. The more realistic computer simulations suggest that MHD instabilities continually occur but only one in many attempts result in a complete reversal of the magnetic dipole field and that these successful reversals often have different morphologies and durations. Computer models have provided new insights into the mechanism of the geodynamo and its reversals, but much better spatial resolution, lower viscosity, and longer simulations are needed to reveal and understand the complex turbulent processes that underlie the polarity reversal phenomenon. |
