Phys. Earth Planet. Inter. http://dx.doi.org/10.1016/j.pepi.2011.10.005 (2011)

Earth's magnetic field is generated by flows within the fluid outer core and is broadly symmetrical about the axis of rotation. Numerical modelling shows that thermal anomalies above the boundary between the core and mantle could cause the magnetic field to deviate from this symmetry.

Hagay Amit and Gaël Choblet at the Université de Nantes, France, numerically simulated the effect of narrow ridges of hot material in the lower mantle on flow within the outer core. Such ridges have been identified beneath central Asia and the Indian Ocean and below the American Pacific coast. Some of the relatively hot ridges seem to act as barriers to the overall westward flow of material at low latitudes in the core, causing it to upwell into elongated zones. The upwelling material causes the overall geomagnetic field to become heterogeneous and asymmetrical.

The researchers suggest that the hot ridges could have caused localized patches of intense geomagnetic flux to the west, below east Asia and the Americas, that should be apparent when averaged over time.