THE CHARACTER OF THE FIELD DURING GEOMAGNETIC REVERSALS

Paleomagnetic evidence shows that polarity transitions are characterized by an interval of directional change and reduced field strength. The best estimates for the duration of the transitional interval average near 4000 years. Both absolute and relative paleointensity determinations suggest that the field intensity drops to 10-22% of its usual value for a site during a reversal. Free decay of the geomagnetic dipole to values this low would take ten times longer than the average transition appears to last, an observation that strongly suggests that the reversal process is not associated with a cessation of convection in the outer core. The transitional field almost certainly involves multipolar sources in the core. The geometry of this field at the earth's surface, however, is the focus of ongoing controversy. A related issue is whether the dipolar part of the transitional field is systematically oriented toward a certain part of the globe. It has been suggested that the field may sometimes change sharply over a period as short as a few weeks. -from Authors