PREFERRED VGP PATHS DURING GEOMAGNETIC POLARITY REVERSALS - SYMMETRY CONSIDERATIONS

The reversing geomagnetic field has been said to have virtual geomagne tic poles (VGPs) confined to a pair of longitudes close to where magne tic flux is concentrated at the core surface today, at approximately 9 0 degrees W and 110 degrees E. This can be explained if flux remains c oncentrated on the same longitudes throughout the transition. The VGP path then depends on site position, reversal sense (R --> N or N --> R ), and sense of flux migration (pole- or equator-ward). For transition al fields which remain antisymmetric about the equator, N --> R transi tions with poleward flux migration give western longitude VGP paths fo r sites in the NE and SW quadrants and eastern longitude VGP paths for sites in the NW and SE quadrants. Sites on quadrant boundaries record rapid, variable VGP positions. Data from the last (Matuyama-Brunhes) reversal are broadly consistent with poleward flux migration.